diff --git a/datasets/openimages.py b/datasets/openimages.py
new file mode 100644
index 0000000000..11aee4d0cb
--- /dev/null
+++ b/datasets/openimages.py
@@ -0,0 +1,165 @@
+import os
+import math
+import json
+from extra.utils import OSX
+import numpy as np
+from PIL import Image
+import pathlib
+import boto3, botocore
+from extra.utils import download_file
+from tqdm import tqdm
+import pandas as pd
+import concurrent.futures
+
+BASEDIR = pathlib.Path(__file__).parent.parent / "datasets/open-images-v6-mlperf"
+BUCKET_NAME = "open-images-dataset"
+BBOX_ANNOTATIONS_URL = "https://storage.googleapis.com/openimages/v5/validation-annotations-bbox.csv"
+MAP_CLASSES_URL = "https://storage.googleapis.com/openimages/v5/class-descriptions-boxable.csv"
+MLPERF_CLASSES = ['Airplane', 'Antelope', 'Apple', 'Backpack', 'Balloon', 'Banana',
+  'Barrel', 'Baseball bat', 'Baseball glove', 'Bee', 'Beer', 'Bench', 'Bicycle',
+  'Bicycle helmet', 'Bicycle wheel', 'Billboard', 'Book', 'Bookcase', 'Boot',
+  'Bottle', 'Bowl', 'Bowling equipment', 'Box', 'Boy', 'Brassiere', 'Bread',
+  'Broccoli', 'Bronze sculpture', 'Bull', 'Bus', 'Bust', 'Butterfly', 'Cabinetry',
+  'Cake', 'Camel', 'Camera', 'Candle', 'Candy', 'Cannon', 'Canoe', 'Carrot', 'Cart',
+  'Castle', 'Cat', 'Cattle', 'Cello', 'Chair', 'Cheese', 'Chest of drawers', 'Chicken',
+  'Christmas tree', 'Coat', 'Cocktail', 'Coffee', 'Coffee cup', 'Coffee table', 'Coin',
+  'Common sunflower', 'Computer keyboard', 'Computer monitor', 'Convenience store',
+  'Cookie', 'Countertop', 'Cowboy hat', 'Crab', 'Crocodile', 'Cucumber', 'Cupboard',
+  'Curtain', 'Deer', 'Desk', 'Dinosaur', 'Dog', 'Doll', 'Dolphin', 'Door', 'Dragonfly',
+  'Drawer', 'Dress', 'Drum', 'Duck', 'Eagle', 'Earrings', 'Egg (Food)', 'Elephant',
+  'Falcon', 'Fedora', 'Flag', 'Flowerpot', 'Football', 'Football helmet', 'Fork',
+  'Fountain', 'French fries', 'French horn', 'Frog', 'Giraffe', 'Girl', 'Glasses',
+  'Goat', 'Goggles', 'Goldfish', 'Gondola', 'Goose', 'Grape', 'Grapefruit', 'Guitar',
+  'Hamburger', 'Handbag', 'Harbor seal', 'Headphones', 'Helicopter', 'High heels',
+  'Hiking equipment', 'Horse', 'House', 'Houseplant', 'Human arm', 'Human beard',
+  'Human body', 'Human ear', 'Human eye', 'Human face', 'Human foot', 'Human hair',
+  'Human hand', 'Human head', 'Human leg', 'Human mouth', 'Human nose', 'Ice cream',
+  'Jacket', 'Jeans', 'Jellyfish', 'Juice', 'Kitchen & dining room table', 'Kite',
+  'Lamp', 'Lantern', 'Laptop', 'Lavender (Plant)', 'Lemon', 'Light bulb', 'Lighthouse',
+  'Lily', 'Lion', 'Lipstick', 'Lizard', 'Man', 'Maple', 'Microphone', 'Mirror',
+  'Mixing bowl', 'Mobile phone', 'Monkey', 'Motorcycle', 'Muffin', 'Mug', 'Mule',
+  'Mushroom', 'Musical keyboard', 'Necklace', 'Nightstand', 'Office building',
+  'Orange', 'Owl', 'Oyster', 'Paddle', 'Palm tree', 'Parachute', 'Parrot', 'Pen',
+  'Penguin', 'Personal flotation device', 'Piano', 'Picture frame', 'Pig', 'Pillow',
+  'Pizza', 'Plate', 'Platter', 'Porch', 'Poster', 'Pumpkin', 'Rabbit', 'Rifle',
+  'Roller skates', 'Rose', 'Salad', 'Sandal', 'Saucer', 'Saxophone', 'Scarf', 'Sea lion',
+  'Sea turtle', 'Sheep', 'Shelf', 'Shirt', 'Shorts', 'Shrimp', 'Sink', 'Skateboard',
+  'Ski', 'Skull', 'Skyscraper', 'Snake', 'Sock', 'Sofa bed', 'Sparrow', 'Spider', 'Spoon',
+  'Sports uniform', 'Squirrel', 'Stairs', 'Stool', 'Strawberry', 'Street light',
+  'Studio couch', 'Suit', 'Sun hat', 'Sunglasses', 'Surfboard', 'Sushi', 'Swan',
+  'Swimming pool', 'Swimwear', 'Tank', 'Tap', 'Taxi', 'Tea', 'Teddy bear', 'Television',
+  'Tent', 'Tie', 'Tiger', 'Tin can', 'Tire', 'Toilet', 'Tomato', 'Tortoise', 'Tower',
+  'Traffic light', 'Train', 'Tripod', 'Truck', 'Trumpet', 'Umbrella', 'Van', 'Vase',
+  'Vehicle registration plate', 'Violin', 'Wall clock', 'Waste container', 'Watch',
+  'Whale', 'Wheel', 'Wheelchair', 'Whiteboard', 'Window', 'Wine', 'Wine glass', 'Woman',
+  'Zebra', 'Zucchini',
+]
+
+def openimages():
+  ann_file = BASEDIR / "validation/labels/openimages-mlperf.json"
+  if not ann_file.is_file():
+    fetch_openimages(ann_file)
+  return ann_file
+
+# this slows down the conversion a lot!
+# maybe use https://raw.githubusercontent.com/scardine/image_size/master/get_image_size.py
+def extract_dims(path): return Image.open(path).size[::-1]
+
+def export_to_coco(class_map, annotations, image_list, dataset_path, output_path, classes=MLPERF_CLASSES):
+  output_path.parent.mkdir(parents=True, exist_ok=True)
+  cats = [{"id": i, "name": c, "supercategory": None} for i, c in enumerate(classes)]
+  categories_map = pd.DataFrame([(i, c) for i, c in enumerate(classes)], columns=["category_id", "category_name"])
+  class_map = class_map.merge(categories_map, left_on="DisplayName", right_on="category_name", how="inner")
+  annotations = annotations[np.isin(annotations["ImageID"], image_list)]
+  annotations = annotations.merge(class_map, on="LabelName", how="inner")
+  annotations["image_id"] = pd.factorize(annotations["ImageID"].tolist())[0]
+  annotations[["height", "width"]] = annotations.apply(lambda x: extract_dims(dataset_path / f"{x['ImageID']}.jpg"), axis=1, result_type="expand")
+
+  # Images
+  imgs = [{"id": int(id + 1), "file_name": f"{image_id}.jpg", "height": row["height"], "width": row["width"], "license": None, "coco_url": None}
+    for (id, image_id), row in (annotations.groupby(["image_id", "ImageID"]).first().iterrows())
+  ]
+
+  # Annotations
+  annots = []
+  for i, row in annotations.iterrows():
+    xmin, ymin, xmax, ymax, img_w, img_h = [row[k] for k in ["XMin", "YMin", "XMax", "YMax", "width", "height"]]
+    x, y, w, h = xmin * img_w, ymin * img_h, (xmax - xmin) * img_w, (ymax - ymin) * img_h
+    coco_annot = {"id": int(i) + 1, "image_id": int(row["image_id"] + 1), "category_id": int(row["category_id"]), "bbox": [x, y, w, h], "area": w * h}
+    coco_annot.update({k: row[k] for k in ["IsOccluded", "IsInside", "IsDepiction", "IsTruncated", "IsGroupOf"]})
+    coco_annot["iscrowd"] = int(row["IsGroupOf"])
+    annots.append(coco_annot)
+
+  info = {"dataset": "openimages_mlperf", "version": "v6"}
+  coco_annotations = {"info": info, "licenses": [], "categories": cats, "images": imgs, "annotations": annots}
+  with open(output_path, "w") as fp:
+    json.dump(coco_annotations, fp)
+
+def get_image_list(class_map, annotations, classes=MLPERF_CLASSES):
+  labels = class_map[np.isin(class_map["DisplayName"], classes)]["LabelName"]
+  image_ids = annotations[np.isin(annotations["LabelName"], labels)]["ImageID"].unique()
+  return image_ids
+
+def download_image(bucket, image_id, data_dir):
+  try:
+    bucket.download_file(f"validation/{image_id}.jpg", f"{data_dir}/{image_id}.jpg")
+  except botocore.exceptions.ClientError as exception:
+    sys.exit(f"ERROR when downloading image `validation/{image_id}`: {str(exception)}")
+
+def fetch_openimages(output_fn):
+  bucket = boto3.resource("s3", config=botocore.config.Config(signature_version=botocore.UNSIGNED)).Bucket(BUCKET_NAME)
+
+  annotations_dir, data_dir = BASEDIR / "annotations", BASEDIR / "validation/data"
+  annotations_dir.mkdir(parents=True, exist_ok=True)
+  data_dir.mkdir(parents=True, exist_ok=True)
+
+  annotations_fn = annotations_dir / BBOX_ANNOTATIONS_URL.split('/')[-1]
+  download_file(BBOX_ANNOTATIONS_URL, annotations_fn)
+  annotations = pd.read_csv(annotations_fn)
+
+  classmap_fn = annotations_dir / MAP_CLASSES_URL.split('/')[-1]
+  download_file(MAP_CLASSES_URL, classmap_fn)
+  class_map = pd.read_csv(classmap_fn, names=["LabelName", "DisplayName"])
+
+  image_list = get_image_list(class_map, annotations)
+
+  with concurrent.futures.ThreadPoolExecutor() as executor:
+    futures = [executor.submit(download_image, bucket, image_id, data_dir) for image_id in image_list]
+    for future in (t := tqdm(concurrent.futures.as_completed(futures), total=len(image_list))):
+      t.set_description(f"Downloading images")
+      future.result()
+
+  print("Converting annotations to COCO format...")
+  export_to_coco(class_map, annotations, image_list, data_dir, output_fn)
+
+def image_load(fn):
+  img_folder = BASEDIR / "validation/data"
+  img = Image.open(img_folder / fn).convert('RGB')
+  import torchvision.transforms.functional as F
+  ret = F.resize(img, size=(800, 800))
+  ret = np.array(ret)
+  return ret, img.size[::-1]
+
+def prepare_target(annotations, img_id, img_size):
+  boxes = [annot["bbox"] for annot in annotations]
+  boxes = np.array(boxes, dtype=np.float32).reshape(-1, 4)
+  boxes[:, 2:] += boxes[:, :2]
+  boxes[:, 0::2] = boxes[:, 0::2].clip(0, img_size[1])
+  boxes[:, 1::2] = boxes[:, 1::2].clip(0, img_size[0])
+  keep = (boxes[:, 3] > boxes[:, 1]) & (boxes[:, 2] > boxes[:, 0])
+  boxes = boxes[keep]
+  classes = [annot["category_id"] for annot in annotations]
+  classes = np.array(classes, dtype=np.int64)
+  classes = classes[keep]
+  return {"boxes": boxes, "labels": classes, "image_id": img_id, "image_size": img_size}
+
+def iterate(coco, bs=8):
+  image_ids = sorted(coco.imgs.keys())
+  for i in range(0, len(image_ids), bs):
+    X, targets  = [], []
+    for img_id in image_ids[i:i+bs]:
+      x, original_size = image_load(coco.loadImgs(img_id)[0]["file_name"])
+      X.append(x)
+      annotations = coco.loadAnns(coco.getAnnIds(img_id))
+      targets.append(prepare_target(annotations, img_id, original_size))
+    yield np.array(X), targets
diff --git a/examples/mlperf/model_eval.py b/examples/mlperf/model_eval.py
index ef66fd6834..bc75b1da83 100644
--- a/examples/mlperf/model_eval.py
+++ b/examples/mlperf/model_eval.py
@@ -42,6 +42,64 @@ def eval_resnet():
     print(f"****** {n}/{d}  {n*100.0/d:.2f}%")
     st = time.perf_counter()
 
+def eval_retinanet():
+  # RetinaNet with ResNeXt50_32X4D
+  from models.resnet import ResNeXt50_32X4D
+  from models.retinanet import RetinaNet
+  mdl = RetinaNet(ResNeXt50_32X4D())
+  mdl.load_from_pretrained()
+
+  input_mean = Tensor([0.485, 0.456, 0.406]).reshape(1, -1, 1, 1)
+  input_std = Tensor([0.229, 0.224, 0.225]).reshape(1, -1, 1, 1)
+  def input_fixup(x):
+    x = x.permute([0,3,1,2]) / 255.0
+    x -= input_mean
+    x /= input_std
+    return x
+
+  from datasets.openimages import openimages, iterate
+  from pycocotools.coco import COCO
+  from pycocotools.cocoeval import COCOeval
+  from contextlib import redirect_stdout
+  coco = COCO(openimages())
+  coco_eval = COCOeval(coco, iouType="bbox")
+  coco_evalimgs, evaluated_imgs, ncats, narea = [], [], len(coco_eval.params.catIds), len(coco_eval.params.areaRng)
+
+  from tinygrad.jit import TinyJit
+  mdlrun = TinyJit(lambda x: mdl(input_fixup(x)).realize())
+
+  n, bs = 0, 8
+  st = time.perf_counter()
+  for x, targets in iterate(coco, bs):
+    dat = Tensor(x.astype(np.float32))
+    mt = time.perf_counter()
+    if dat.shape[0] == bs:
+      outs = mdlrun(dat).numpy()
+    else:
+      mdlrun.jit_cache = None
+      outs =  mdl(input_fixup(dat)).numpy()
+    et = time.perf_counter()
+    predictions = mdl.postprocess_detections(outs, input_size=dat.shape[1:3], orig_image_sizes=[t["image_size"] for t in targets])
+    ext = time.perf_counter()
+    n += len(targets)
+    print(f"[{n}/{len(coco.imgs)}] == {(mt-st)*1000:.2f} ms loading data, {(et-mt)*1000:.2f} ms to run model, {(ext-et)*1000:.2f} ms for postprocessing")
+    img_ids = [t["image_id"] for t in targets]
+    coco_results  = [{"image_id": targets[i]["image_id"], "category_id": label, "bbox": box, "score": score} 
+      for i, prediction in enumerate(predictions) for box, score, label in zip(*prediction.values())]
+    with redirect_stdout(None):
+      coco_eval.cocoDt = coco.loadRes(coco_results)
+      coco_eval.params.imgIds = img_ids
+      coco_eval.evaluate()
+    evaluated_imgs.extend(img_ids)
+    coco_evalimgs.append(np.array(coco_eval.evalImgs).reshape(ncats, narea, len(img_ids)))
+    st = time.perf_counter()
+
+  coco_eval.params.imgIds = evaluated_imgs
+  coco_eval._paramsEval.imgIds = evaluated_imgs
+  coco_eval.evalImgs = list(np.concatenate(coco_evalimgs, -1).flatten())
+  coco_eval.accumulate()
+  coco_eval.summarize()
+
 def eval_rnnt():
   # RNN-T
   from models.rnnt import RNNT
diff --git a/examples/mlperf/model_spec.py b/examples/mlperf/model_spec.py
index 6b7c37a9e4..f11f76eb05 100644
--- a/examples/mlperf/model_spec.py
+++ b/examples/mlperf/model_spec.py
@@ -17,8 +17,12 @@ def spec_resnet():
   test_model(mdl, img)
 
 def spec_retinanet():
-  # TODO: Retinanet
-  pass
+  # Retinanet with ResNet backbone
+  from models.resnet import ResNet50
+  from models.retinanet import RetinaNet
+  mdl = RetinaNet(ResNet50(), num_classes=91, num_anchors=9)
+  img = Tensor.randn(1, 3, 224, 224)
+  test_model(mdl, img)
 
 def spec_unet3d():
   # 3D UNET
diff --git a/models/resnet.py b/models/resnet.py
index 0712274ef9..8bc955a35f 100644
--- a/models/resnet.py
+++ b/models/resnet.py
@@ -5,7 +5,8 @@ from extra.utils import get_child
 class BasicBlock:
   expansion = 1
 
-  def __init__(self, in_planes, planes, stride=1):
+  def __init__(self, in_planes, planes, stride=1, groups=1, base_width=64):
+    assert groups == 1 and base_width == 64, "BasicBlock only supports groups=1 and base_width=64"
     self.conv1 = nn.Conv2d(in_planes, planes, kernel_size=3, stride=stride, padding=1, bias=False)
     self.bn1 = nn.BatchNorm2d(planes)
     self.conv2 = nn.Conv2d(planes, planes, kernel_size=3, padding=1, stride=1, bias=False)
@@ -29,12 +30,13 @@ class Bottleneck:
   # NOTE: the original implementation places stride at the first convolution (self.conv1), this is the v1.5 variant
   expansion = 4
 
-  def __init__(self, in_planes, planes, stride=1):
-    self.conv1 = nn.Conv2d(in_planes, planes, kernel_size=1, bias=False)
-    self.bn1 = nn.BatchNorm2d(planes)
-    self.conv2 = nn.Conv2d(planes, planes, kernel_size=3, padding=1, stride=stride, bias=False)
-    self.bn2 = nn.BatchNorm2d(planes)
-    self.conv3 = nn.Conv2d(planes, self.expansion*planes, kernel_size=1, bias=False)
+  def __init__(self, in_planes, planes, stride=1, groups=1, base_width=64):
+    width = int(planes * (base_width / 64.0)) * groups
+    self.conv1 = nn.Conv2d(in_planes, width, kernel_size=1, bias=False)
+    self.bn1 = nn.BatchNorm2d(width)
+    self.conv2 = nn.Conv2d(width, width, kernel_size=3, padding=1, stride=stride, groups=groups, bias=False)
+    self.bn2 = nn.BatchNorm2d(width)
+    self.conv3 = nn.Conv2d(width, self.expansion*planes, kernel_size=1, bias=False)
     self.bn3 = nn.BatchNorm2d(self.expansion*planes)
     self.downsample = []
     if stride != 1 or in_planes != self.expansion*planes:
@@ -52,7 +54,7 @@ class Bottleneck:
     return out
 
 class ResNet:
-  def __init__(self, num, num_classes):
+  def __init__(self, num, num_classes, groups=1, width_per_group=64):
     self.num = num
 
     self.block = {
@@ -73,6 +75,8 @@ class ResNet:
 
     self.in_planes = 64
 
+    self.groups = groups
+    self.base_width = width_per_group
     self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, bias=False, padding=3)
     self.bn1 = nn.BatchNorm2d(64)
     self.layer1 = self._make_layer(self.block, 64, self.num_blocks[0], stride=1)
@@ -85,7 +89,7 @@ class ResNet:
     strides = [stride] + [1] * (num_blocks-1)
     layers = []
     for stride in strides:
-      layers.append(block(self.in_planes, planes, stride))
+      layers.append(block(self.in_planes, planes, stride, self.groups, self.base_width))
       self.in_planes = planes * block.expansion
     return layers
 
@@ -107,14 +111,15 @@ class ResNet:
     # TODO replace with fake torch load
 
     model_urls = {
-      18: 'https://download.pytorch.org/models/resnet18-5c106cde.pth',
-      34: 'https://download.pytorch.org/models/resnet34-333f7ec4.pth',
-      50: 'https://download.pytorch.org/models/resnet50-19c8e357.pth',
-      101: 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth',
-      152: 'https://download.pytorch.org/models/resnet152-b121ed2d.pth'
+      (18, 1, 64): 'https://download.pytorch.org/models/resnet18-5c106cde.pth',
+      (34, 1, 64): 'https://download.pytorch.org/models/resnet34-333f7ec4.pth',
+      (50, 1, 64): 'https://download.pytorch.org/models/resnet50-19c8e357.pth',
+      (50, 32, 4): 'https://download.pytorch.org/models/resnext50_32x4d-7cdf4587.pth',
+      (101, 1, 64): 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth',
+      (152, 1, 64): 'https://download.pytorch.org/models/resnet152-b121ed2d.pth',
     }
 
-    self.url = model_urls[self.num]
+    self.url = model_urls[(self.num, self.groups, self.base_width)]
 
     from torch.hub import load_state_dict_from_url
     state_dict = load_state_dict_from_url(self.url, progress=True)
@@ -126,7 +131,8 @@ class ResNet:
         print("skipping fully connected layer")
         continue # Skip FC if transfer learning
 
-      assert obj.shape == dat.shape, (k, obj.shape, dat.shape)
+      # TODO: remove or when #777 is merged
+      assert obj.shape == dat.shape or (obj.shape == (1,) and dat.shape == ()), (k, obj.shape, dat.shape)
       obj.assign(dat)
 
 ResNet18 = lambda num_classes=1000: ResNet(18, num_classes=num_classes)
@@ -134,3 +140,4 @@ ResNet34 = lambda num_classes=1000: ResNet(34, num_classes=num_classes)
 ResNet50 = lambda num_classes=1000: ResNet(50, num_classes=num_classes)
 ResNet101 = lambda num_classes=1000: ResNet(101, num_classes=num_classes)
 ResNet152 = lambda num_classes=1000: ResNet(152, num_classes=num_classes)
+ResNeXt50_32X4D = lambda num_classes=1000: ResNet(50, num_classes=num_classes, groups=32, width_per_group=4)
diff --git a/models/retinanet.py b/models/retinanet.py
new file mode 100644
index 0000000000..8baf0f073e
--- /dev/null
+++ b/models/retinanet.py
@@ -0,0 +1,237 @@
+import math
+from tinygrad.helpers import flatten
+import tinygrad.nn as nn
+from models.resnet import ResNet
+from extra.utils import get_child
+import numpy as np
+
+def nms(boxes, scores, thresh=0.5):
+  x1, y1, x2, y2 = np.rollaxis(boxes, 1)
+  areas = (x2 - x1 + 1) * (y2 - y1 + 1)
+  to_process, keep = scores.argsort()[::-1], []
+  while to_process.size > 0:
+    cur, to_process = to_process[0], to_process[1:]
+    keep.append(cur)
+    inter_x1 = np.maximum(x1[cur], x1[to_process])
+    inter_y1 = np.maximum(y1[cur], y1[to_process])
+    inter_x2 = np.minimum(x2[cur], x2[to_process])
+    inter_y2 = np.minimum(y2[cur], y2[to_process])
+    inter_area = np.maximum(0, inter_x2 - inter_x1 + 1) * np.maximum(0, inter_y2 - inter_y1 + 1)
+    iou = inter_area / (areas[cur] + areas[to_process] - inter_area)
+    to_process = to_process[np.where(iou <= thresh)[0]]
+  return keep
+
+def decode_bbox(offsets, anchors):
+  dx, dy, dw, dh = np.rollaxis(offsets, 1)
+  widths, heights = anchors[:, 2] - anchors[:, 0], anchors[:, 3] - anchors[:, 1]
+  cx, cy = anchors[:, 0] + 0.5 * widths, anchors[:, 1] + 0.5 * heights
+  pred_cx, pred_cy = dx * widths + cx, dy * heights + cy
+  pred_w, pred_h = np.exp(dw) * widths, np.exp(dh) * heights
+  pred_x1, pred_y1 = pred_cx - 0.5 * pred_w, pred_cy - 0.5 * pred_h
+  pred_x2, pred_y2 = pred_cx + 0.5 * pred_w, pred_cy + 0.5 * pred_h
+  return np.stack([pred_x1, pred_y1, pred_x2, pred_y2], axis=1, dtype=np.float32)
+
+def generate_anchors(input_size, grid_sizes, scales, aspect_ratios):
+  assert len(scales) == len(aspect_ratios) == len(grid_sizes)
+  anchors = []
+  for s, ar, gs in zip(scales, aspect_ratios, grid_sizes):
+    s, ar = np.array(s), np.array(ar)
+    h_ratios = np.sqrt(ar)
+    w_ratios = 1 / h_ratios
+    ws = (w_ratios[:, None] * s[None, :]).reshape(-1)
+    hs = (h_ratios[:, None] * s[None, :]).reshape(-1)
+    base_anchors = (np.stack([-ws, -hs, ws, hs], axis=1) / 2).round()
+    stride_h, stride_w = input_size[0] // gs[0], input_size[1] // gs[1]
+    shifts_x, shifts_y = np.meshgrid(np.arange(gs[1]) * stride_w, np.arange(gs[0]) * stride_h)
+    shifts_x = shifts_x.reshape(-1)
+    shifts_y = shifts_y.reshape(-1)
+    shifts = np.stack([shifts_x, shifts_y, shifts_x, shifts_y], axis=1, dtype=np.float32)
+    anchors.append((shifts[:, None] + base_anchors[None, :]).reshape(-1, 4))
+  return anchors
+
+class RetinaNet:
+  def __init__(self, backbone: ResNet, num_classes=264, num_anchors=9, scales=None, aspect_ratios=None):
+    assert isinstance(backbone, ResNet)
+    scales = tuple((i, int(i*2**(1/3)), int(i*2**(2/3))) for i in 2**np.arange(5, 10)) if scales is None else scales
+    aspect_ratios = ((0.5, 1.0, 2.0),) * len(scales) if aspect_ratios is None else aspect_ratios
+    self.num_anchors, self.num_classes = num_anchors, num_classes
+    assert len(scales) == len(aspect_ratios) and all([self.num_anchors == len(s) * len(ar) for s, ar in zip(scales, aspect_ratios)])
+
+    self.backbone = ResNetFPN(backbone)
+    self.head = RetinaHead(self.backbone.out_channels, num_anchors=num_anchors, num_classes=num_classes)
+    self.anchor_gen = lambda input_size: generate_anchors(input_size, self.backbone.compute_grid_sizes(input_size), scales, aspect_ratios)
+
+  def __call__(self, x):
+    return self.forward(x)
+  def forward(self, x):
+    return self.head(self.backbone(x))
+
+  def load_from_pretrained(self):
+    model_urls = {
+      (50, 1, 64): "https://download.pytorch.org/models/retinanet_resnet50_fpn_coco-eeacb38b.pth",
+      (50, 32, 4): "https://zenodo.org/record/6605272/files/retinanet_model_10.zip",
+    }
+    self.url = model_urls[(self.backbone.body.num, self.backbone.body.groups, self.backbone.body.base_width)]
+    from torch.hub import load_state_dict_from_url
+    state_dict = load_state_dict_from_url(self.url, progress=True, map_location='cpu')
+    state_dict = state_dict['model'] if 'model' in state_dict.keys() else state_dict
+    for k, v in state_dict.items():
+      obj = get_child(self, k)
+      dat = v.detach().numpy()
+      assert obj.shape == dat.shape, (k, obj.shape, dat.shape)
+      obj.assign(dat)
+
+  # predictions: (BS, (H1W1+...+HmWm)A, 4 + K)
+  def postprocess_detections(self, predictions, input_size=(800, 800), image_sizes=None, orig_image_sizes=None, score_thresh=0.05, topk_candidates=1000, nms_thresh=0.5):
+    anchors = self.anchor_gen(input_size)
+    grid_sizes = self.backbone.compute_grid_sizes(input_size)
+    split_idx = np.cumsum([int(self.num_anchors * sz[0] * sz[1]) for sz in grid_sizes[:-1]])
+    detections = []
+    for i, predictions_per_image in enumerate(predictions):
+      h, w = input_size if image_sizes is None else image_sizes[i]
+
+      predictions_per_image = np.split(predictions_per_image, split_idx)
+      offsets_per_image = [br[:, :4] for br in predictions_per_image]
+      scores_per_image = [cl[:, 4:] for cl in predictions_per_image]
+
+      image_boxes, image_scores, image_labels = [], [], []
+      for offsets_per_level, scores_per_level, anchors_per_level in zip(offsets_per_image, scores_per_image, anchors):
+        # remove low scoring boxes
+        scores_per_level = scores_per_level.flatten()
+        keep_idxs = scores_per_level > score_thresh
+        scores_per_level = scores_per_level[keep_idxs]
+
+        # keep topk
+        topk_idxs = np.where(keep_idxs)[0]
+        num_topk = min(len(topk_idxs), topk_candidates)
+        sort_idxs = scores_per_level.argsort()[-num_topk:][::-1]
+        topk_idxs, scores_per_level = topk_idxs[sort_idxs], scores_per_level[sort_idxs]
+
+        # bbox coords from offsets
+        anchor_idxs = topk_idxs // self.num_classes
+        labels_per_level = topk_idxs % self.num_classes
+        boxes_per_level = decode_bbox(offsets_per_level[anchor_idxs], anchors_per_level[anchor_idxs])
+        # clip to image size
+        clipped_x = boxes_per_level[:, 0::2].clip(0, w)
+        clipped_y = boxes_per_level[:, 1::2].clip(0, h)
+        boxes_per_level = np.stack([clipped_x, clipped_y], axis=2).reshape(-1, 4)
+
+        image_boxes.append(boxes_per_level)
+        image_scores.append(scores_per_level)
+        image_labels.append(labels_per_level)
+
+      image_boxes = np.concatenate(image_boxes)
+      image_scores = np.concatenate(image_scores)
+      image_labels = np.concatenate(image_labels)
+
+      # nms for each class
+      keep_mask = np.zeros_like(image_scores, dtype=bool)
+      for class_id in np.unique(image_labels):
+        curr_indices = np.where(image_labels == class_id)[0]
+        curr_keep_indices = nms(image_boxes[curr_indices], image_scores[curr_indices], nms_thresh)
+        keep_mask[curr_indices[curr_keep_indices]] = True
+      keep = np.where(keep_mask)[0]
+      keep = keep[image_scores[keep].argsort()[::-1]]
+
+      # resize bboxes back to original size
+      image_boxes = image_boxes[keep]
+      if orig_image_sizes is not None:
+        resized_x = image_boxes[:, 0::2] * orig_image_sizes[i][1] / w
+        resized_y = image_boxes[:, 1::2] * orig_image_sizes[i][0] / h
+        image_boxes = np.stack([resized_x, resized_y], axis=2).reshape(-1, 4)
+      # xywh format
+      image_boxes = np.concatenate([image_boxes[:, :2], image_boxes[:, 2:] - image_boxes[:, :2]], axis=1)
+
+      detections.append({"boxes":image_boxes, "scores":image_scores[keep], "labels":image_labels[keep]})
+    return detections
+
+class ClassificationHead:
+  def __init__(self, in_channels, num_anchors, num_classes):
+    self.num_classes = num_classes
+    self.conv = flatten([(nn.Conv2d(in_channels, in_channels, kernel_size=3, padding=1), lambda x: x.relu()) for _ in range(4)])
+    self.cls_logits = nn.Conv2d(in_channels, num_anchors * num_classes, kernel_size=3, padding=1)
+  def __call__(self, x):
+    out = [self.cls_logits(feat.sequential(self.conv)).permute(0, 2, 3, 1).reshape(feat.shape[0], -1, self.num_classes) for feat in x]
+    return out[0].cat(*out[1:], dim=1).sigmoid()
+
+class RegressionHead:
+  def __init__(self, in_channels, num_anchors):
+    self.conv = flatten([(nn.Conv2d(in_channels, in_channels, kernel_size=3, padding=1), lambda x: x.relu()) for _ in range(4)])
+    self.bbox_reg = nn.Conv2d(in_channels, num_anchors * 4, kernel_size=3, padding=1)
+  def __call__(self, x):
+    out = [self.bbox_reg(feat.sequential(self.conv)).permute(0, 2, 3, 1).reshape(feat.shape[0], -1, 4) for feat in x]
+    return out[0].cat(*out[1:], dim=1)
+
+class RetinaHead:
+  def __init__(self, in_channels, num_anchors, num_classes):
+    self.classification_head = ClassificationHead(in_channels, num_anchors, num_classes)
+    self.regression_head = RegressionHead(in_channels, num_anchors)
+  def __call__(self, x):
+    pred_bbox, pred_class = self.regression_head(x), self.classification_head(x)
+    out = pred_bbox.cat(pred_class, dim=-1)
+    return out
+
+class ResNetFPN:
+  def __init__(self, resnet, out_channels=256, returned_layers=[2, 3, 4]):
+    self.out_channels = out_channels
+    self.body = resnet
+    in_channels_list = [(self.body.in_planes // 8) * 2 ** (i - 1) for i in returned_layers]
+    self.fpn = FPN(in_channels_list, out_channels)
+
+  # this is needed to decouple inference from postprocessing (anchors generation)
+  def compute_grid_sizes(self, input_size):
+    return np.ceil(np.array(input_size)[None, :] / 2 ** np.arange(3, 8)[:, None])
+
+  def __call__(self, x):
+    out = self.body.bn1(self.body.conv1(x)).relu()
+    out = out.pad2d([1,1,1,1]).max_pool2d((3,3), 2)
+    out = out.sequential(self.body.layer1)
+    p3 = out.sequential(self.body.layer2)
+    p4 = p3.sequential(self.body.layer3)
+    p5 = p4.sequential(self.body.layer4)
+    return self.fpn([p3, p4, p5])
+
+class ExtraFPNBlock:
+  def __init__(self, in_channels, out_channels):
+    self.p6 = nn.Conv2d(in_channels, out_channels, kernel_size=3, stride=2, padding=1)
+    self.p7 = nn.Conv2d(out_channels, out_channels, kernel_size=3, stride=2, padding=1)
+    self.use_P5 = in_channels == out_channels
+
+  def __call__(self, p, c):
+    p5, c5 = p[-1], c[-1]
+    x = p5 if self.use_P5 else c5
+    p6 = self.p6(x)
+    p7 = self.p7(p6.relu())
+    p.extend([p6, p7])
+    return p
+
+class FPN:
+  def __init__(self, in_channels_list, out_channels, extra_blocks=None):
+    self.inner_blocks, self.layer_blocks = [], []
+    for in_channels in in_channels_list:
+      self.inner_blocks.append(nn.Conv2d(in_channels, out_channels, kernel_size=1))
+      self.layer_blocks.append(nn.Conv2d(out_channels, out_channels, kernel_size=3, padding=1))
+    self.extra_blocks = ExtraFPNBlock(256, 256) if extra_blocks is None else extra_blocks
+
+  def __call__(self, x):
+    last_inner = self.inner_blocks[-1](x[-1])
+    results = [self.layer_blocks[-1](last_inner)]
+    for idx in range(len(x) - 2, -1, -1):
+      inner_lateral = self.inner_blocks[idx](x[idx])
+
+      # upsample to inner_lateral's shape
+      (ih, iw), (oh, ow), prefix = last_inner.shape[-2:], inner_lateral.shape[-2:], last_inner.shape[:-2]
+      eh, ew = math.ceil(oh / ih), math.ceil(ow / iw)
+      inner_top_down = last_inner.reshape(*prefix, ih, 1, iw, 1).expand(*prefix, ih, eh, iw, ew).reshape(*prefix, ih*eh, iw*ew)[:, :, :oh, :ow]
+
+      last_inner = inner_lateral + inner_top_down
+      results.insert(0, self.layer_blocks[idx](last_inner))
+    if self.extra_blocks is not None:
+      results = self.extra_blocks(results, x)
+    return results
+
+if __name__ == "__main__":
+  from models.resnet import ResNeXt50_32X4D
+  backbone = ResNeXt50_32X4D()
+  retina = RetinaNet(backbone)
+  retina.load_from_pretrained()