fix error: TypeError: Object of type Tensor is not JSON serializable … (#29568)

fix error: TypeError: Object of type Tensor is not JSON serializable trainer
Co-authored-by: Zach Mueller <muellerzr@gmail.com>

src/transformers/trainer.py

@@ -2415,7 +2415,7 @@ class Trainer:
 
             logs["loss"] = round(tr_loss_scalar / (self.state.global_step - self._globalstep_last_logged), 4)
             if grad_norm is not None:
-                logs["grad_norm"] = grad_norm.item() if torch.is_tensor(grad_norm) else grad_norm
+                logs["grad_norm"] = grad_norm.detach().item() if isinstance(grad_norm, torch.Tensor) else grad_norm
             logs["learning_rate"] = self._get_learning_rate()
 
             self._total_loss_scalar += tr_loss_scalar