Add an error message that fires when Reformer is not in training mode, but one...

Add an error message that fires when Reformer is not in training mode, but one runs .backward() (#11117)

src/transformers/models/reformer/modeling_reformer.py

@@ -1512,6 +1512,10 @@ class ReformerLayer(nn.Module):
         # Implementation of RevNet (see Fig. 6 in https://towardsdatascience.com/illustrating-the-reformer-393575ac6ba0)
         # This code is heavily inspired by https://github.com/lucidrains/reformer-pytorch/blob/master/reformer_pytorch/reversible.py
 
+        assert (
+            self.training
+        ), "If you want to train `ReformerModel` and its variations, make sure to use `model.train()` to put the model into training mode."
+
         with torch.enable_grad():
             next_attn_output.requires_grad = True