RetinaNet losses (#9536)

* add sigmoid_focal_loss and l1_loss

* update ref implementation comment

examples/mlperf/losses.py

@@ -1,6 +1,29 @@
 from examples.mlperf.metrics import dice_score
+from tinygrad import Tensor
 
 def dice_ce_loss(pred, tgt):
   ce = pred.permute(0, 2, 3, 4, 1).sparse_categorical_crossentropy(tgt.squeeze(1))
   dice = (1.0 - dice_score(pred, tgt, argmax=False, to_one_hot_x=False)).mean()
   return (dice + ce) / 2
+
+def sigmoid_focal_loss(pred:Tensor, tgt:Tensor, alpha:float=0.25, gamma:float=2.0, reduction:str="none") -> Tensor:
+   assert reduction in ["mean", "sum", "none"], f"unsupported reduction {reduction}"
+   p, ce_loss = pred.sigmoid(), pred.binary_crossentropy_logits(tgt, reduction="none")
+   p_t = p * tgt + (1 - p) * (1 - tgt)
+   loss = ce_loss * ((1 - p_t) ** gamma)
+
+   if alpha >= 0:
+     alpha_t = alpha * tgt + (1 - alpha) * (1 - tgt)
+     loss = loss * alpha_t
+
+   if reduction == "mean": loss = loss.mean()
+   elif reduction == "sum": loss = loss.sum()
+   return loss
+
+def l1_loss(pred:Tensor, tgt:Tensor, reduction:str="none") -> Tensor:
+  assert reduction in ["mean", "sum", "none"], f"unsupported reduction {reduction}"
+  loss = (pred - tgt).abs()
+
+  if reduction == "mean": loss = loss.mean()
+  elif reduction == "sum": loss = loss.sum()
+  return loss

test/external/external_test_losses.py

@@ -1,20 +1,40 @@
 from tinygrad import Tensor
+from test.external.mlperf_retinanet.focal_loss import sigmoid_focal_loss as ref_sigmoid_focal_loss
 from test.external.mlperf_unet3d.dice import DiceCELoss
-from examples.mlperf.losses import dice_ce_loss
+from examples.mlperf.losses import dice_ce_loss, sigmoid_focal_loss, l1_loss
 
 import numpy as np
 import torch
 import unittest
 
 class ExternalTestLosses(unittest.TestCase):
-  def _test_losses(self, tinygrad_metrics, orig_metrics, pred, label):
-    tinygrad_metrics_res = tinygrad_metrics(Tensor(pred), Tensor(label)).numpy()
-    orig_metrics_res = orig_metrics(torch.from_numpy(pred), torch.from_numpy(label)).numpy()
-    np.testing.assert_allclose(tinygrad_metrics_res, orig_metrics_res, atol=1e-4)
+  def setUp(self):
+    np.random.seed(42)
 
-  def test_dice_ce(self):
+  def _assert_loss(self, pred, tgt, tinygrad_metrics, ref_metrics, rtol=1e-07, atol=0, **kwargs):
+    tinygrad_metrics_res = tinygrad_metrics(Tensor(pred), Tensor(tgt), **kwargs)
+    ref_metrics_res = ref_metrics(torch.from_numpy(pred), torch.from_numpy(tgt), **kwargs)
+    np.testing.assert_allclose(tinygrad_metrics_res.numpy(), ref_metrics_res.numpy(), rtol=rtol, atol=atol)
+
+  def test_dice_ce_loss(self):
     pred, label = np.random.rand(1, 3, 128, 128, 128).astype(np.float32), np.ones((1, 1, 128, 128, 128)).astype(np.uint8)
-    self._test_losses(dice_ce_loss, DiceCELoss(True, True, "NCDHW", False), pred, label)
+    tinygrad_metrics_res, ref_metrics_res = dice_ce_loss, DiceCELoss(True, True, "NCDHW", False)
+    self._assert_loss(pred, label, tinygrad_metrics_res, ref_metrics_res, atol=1e-4)
+
+  def test_sigmoid_focal_loss(self):
+    def _apply_logit(p): return np.log(p / (1 - p))
+    pred, tgt = _apply_logit(np.random.rand(5,2).astype(np.float32)), np.random.randint(0, 2, (5, 2)).astype(np.float32)
+    for reduction in ["mean", "sum", "none"]:
+      for alpha, gamma in zip([-1, 0.58], [0, 2]):
+        self._assert_loss(pred, tgt, sigmoid_focal_loss, ref_sigmoid_focal_loss, rtol=1e-4, alpha=alpha, gamma=gamma, reduction=reduction)
+
+  def test_l1_loss(self):
+    N, C, H, W = 3, 4, 5, 6
+    shapes = ((N, C), (N, C, H), (N, C, H, W))
+    for reduction in ["mean", "sum", "none"]:
+      for shape in shapes:
+        pred, tgt = np.random.randint(shape).astype(np.float32), np.random.randint(shape)
+        self._assert_loss(pred, tgt, l1_loss, torch.nn.functional.l1_loss, reduction=reduction)
 
 if __name__ == '__main__':
   unittest.main()

test/external/mlperf_retinanet/focal_loss.py

@@ -0,0 +1,51 @@
+# Copied from https://github.com/mlcommons/training/blob/cdd928d4596c142c15a7d86b2eeadbac718c8da2/single_stage_detector/ssd/model/focal_loss.py
+
+import torch
+import torch.nn.functional as F
+
+
+def sigmoid_focal_loss(
+    inputs: torch.Tensor,
+    targets: torch.Tensor,
+    alpha: float = 0.25,
+    gamma: float = 2,
+    reduction: str = "none",
+):
+    """
+    Original implementation from https://github.com/facebookresearch/fvcore/blob/master/fvcore/nn/focal_loss.py .
+    Loss used in RetinaNet for dense detection: https://arxiv.org/abs/1708.02002.
+
+    Args:
+        inputs: A float tensor of arbitrary shape.
+                The predictions for each example.
+        targets: A float tensor with the same shape as inputs. Stores the binary
+                classification label for each element in inputs
+                (0 for the negative class and 1 for the positive class).
+        alpha: (optional) Weighting factor in range (0,1) to balance
+                positive vs negative examples or -1 for ignore. Default = 0.25
+        gamma: Exponent of the modulating factor (1 - p_t) to
+               balance easy vs hard examples.
+        reduction: 'none' | 'mean' | 'sum'
+                 'none': No reduction will be applied to the output.
+                 'mean': The output will be averaged.
+                 'sum': The output will be summed.
+    Returns:
+        Loss tensor with the reduction option applied.
+    """
+    p = torch.sigmoid(inputs)
+    ce_loss = F.binary_cross_entropy_with_logits(
+        inputs, targets, reduction="none"
+    )
+    p_t = p * targets + (1 - p) * (1 - targets)
+    loss = ce_loss * ((1 - p_t) ** gamma)
+
+    if alpha >= 0:
+        alpha_t = alpha * targets + (1 - alpha) * (1 - targets)
+        loss = alpha_t * loss
+
+    if reduction == "mean":
+        loss = loss.mean()
+    elif reduction == "sum":
+        loss = loss.sum()
+
+    return loss