Neuron: When save_safetensor=False, no need to move model to CPU (#29703)

save_safetensor=True is default as of release 4.35.0, which then
required TPU hotfix https://github.com/huggingface/transformers/pull/27799
(issue https://github.com/huggingface/transformers/issues/27578).
However, when the flag save_safetensor is set to False (compatibility mode),
moving the model to CPU causes generation of too many graphs
during checkpoint https://github.com/huggingface/transformers/issues/28438.
This PR disable moving of model to CPU when save_safetensor=False.

src/transformers/trainer.py

@@ -3267,7 +3267,8 @@ class Trainer:
         logger.info(f"Saving model checkpoint to {output_dir}")
         model = self.model
         xm.mark_step()
-        model.to("cpu")
+        if self.args.save_safetensors:
+            model.to("cpu")
 
         if xm.is_master_ordinal():
             os.makedirs(output_dir, exist_ok=True)
@@ -3302,7 +3303,8 @@ class Trainer:
 
         # We moved the model from TPU -> CPU for saving the weights.
         # Now we should move it back to subsequent compute still works.
-        model.to(self.args.device)
+        if self.args.save_safetensors:
+            model.to(self.args.device)
 
     def _save(self, output_dir: Optional[str] = None, state_dict=None):
         # If we are executing this function, we are the process zero, so we don't check for that.