[MLPerf] UNet3D dataloader (#4343)

* add support for train/val datasets for kits19

* split dataset into train and val sets

* add tests for kits19 dataloader

* add MLPerf dataset tests to CI

* update unet3d model_eval script

* fix linting

* add nibabel

* fix how mock dataset gets created

* update ref implementation with permalink and no edits

* clean up test and update rand_flip implementation

* cleanups

setup.py

@@ -53,6 +53,7 @@ setup(name='tinygrad',
             "librosa",
             "networkx",
             "hypothesis",
+            "nibabel",
         ],
         'docs': [
             "mkdocs-material",