fixed trainer tr_loss memory leak (#6999)

* fixed trainer tr_loss memory leak

* detached returned training loss from computation graph in the Trainer class' training_step() method

* Revert "fixed trainer tr_loss memory leak"

This reverts commit 47226e4e

src/transformers/trainer.py

@@ -1024,7 +1024,7 @@ class Trainer:
         else:
             loss.backward()
 
-        return loss
+        return loss.detach()
 
     def is_local_master(self) -> bool:
         """