add easy tensor shape match test

src/transformers/modeling_utils.py

@@ -923,7 +923,10 @@ class PreTrainedModel(nn.Module):
                 for layer_past in past:
                     # copy the relevant beam idx past to past
                     reordered_layer_past = [layer_past[:, i].unsqueeze(1).clone().detach() for i in beam_idx]
-                    reordered_past.append(torch.cat(reordered_layer_past, dim=1))
+                    reordered_layer_past = torch.cat(reordered_layer_past, dim=1)
+                    # check that shape matches
+                    assert reordered_layer_past.shape == layer_past.shape
+                    reordered_past.append(reordered_layer_past)
                 past = tuple(reordered_past)
 
             # update current length