return total grad norm in torchrec grad clipping (#2507)

Summary:
Pull Request resolved: #2507

this is to keep consistent with torch.nn.utils.clip_grad_norm_

Reviewed By: awgu

Differential Revision: D64712277

fbshipit-source-id: 689e02bd21dc37568c3347d5b0833c573f042c15

Author: weifengpy

torchrec/optim/clipping.py

@@ -136,7 +136,7 @@ def step(self, closure: Any = None) -> None:
         self._step_num += 1
 
     @torch.no_grad()
-    def clip_grad_norm_(self) -> None:
+    def clip_grad_norm_(self) -> Optional[Union[float, torch.Tensor]]:
         """Clip the gradient norm of all parameters."""
         max_norm = self._max_gradient
         norm_type = float(self._norm_type)
@@ -224,6 +224,7 @@ def clip_grad_norm_(self) -> None:
         clip_coef = cast(torch.Tensor, max_norm / (total_grad_norm + 1e-6))
         clip_coef_clamped = torch.clamp(clip_coef, max=1.0)
         torch._foreach_mul_(all_grads, clip_coef_clamped)
+        return total_grad_norm
 
 
 def _batch_cal_norm(