Remove hook torch.nan_to_num(x) (ultralytics#8826)

* Remove hook `torch.nan_to_num(x)`

Observed erratic training behavior (green line) with the nan_to_num hook in classifier branch. I'm going to remove it from master.

* Update train.py

train.py

@@ -131,7 +131,7 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
     freeze = [f'model.{x}.' for x in (freeze if len(freeze) > 1 else range(freeze[0]))]  # layers to freeze
     for k, v in model.named_parameters():
         v.requires_grad = True  # train all layers
-        v.register_hook(lambda x: torch.nan_to_num(x))  # NaN to 0.0
+        # v.register_hook(lambda x: torch.nan_to_num(x))  # NaN to 0 (commented for erratic training results)
         if any(x in k for x in freeze):
             LOGGER.info(f'freezing {k}')
             v.requires_grad = False