Ensure PT model is in evaluation mode and lightweight forward pass done (#17970)

src/transformers/commands/pt_to_tf.py

@@ -145,7 +145,7 @@ class PTtoTFCommand(BaseTransformersCLICommand):
             # If the current attribute is a tensor, it is a leaf and we make the comparison. Otherwise, we will dig in
             # recursivelly, keeping the name of the attribute.
             if isinstance(pt_out, torch.Tensor):
-                tensor_difference = np.max(np.abs(pt_out.detach().numpy() - tf_out.numpy()))
+                tensor_difference = np.max(np.abs(pt_out.numpy() - tf_out.numpy()))
                 differences[attr_name] = tensor_difference
             else:
                 root_name = attr_name
@@ -270,9 +270,13 @@ class PTtoTFCommand(BaseTransformersCLICommand):
 
         # Load models and acquire a basic input compatible with the model.
         pt_model = pt_class.from_pretrained(self._local_dir)
+        pt_model.eval()
+
         tf_from_pt_model = tf_class.from_pretrained(self._local_dir, from_pt=True)
         pt_input, tf_input = self.get_inputs(pt_model, config)
-        pt_outputs = pt_model(**pt_input, output_hidden_states=True)
+
+        with torch.no_grad():
+            pt_outputs = pt_model(**pt_input, output_hidden_states=True)
         del pt_model  # will no longer be used, and may have a large memory footprint
 
         tf_from_pt_model = tf_class.from_pretrained(self._local_dir, from_pt=True)