add prepare inputs for transfo_xl and xlnet

src/transformers/modeling_transfo_xl.py

@@ -930,3 +930,12 @@ class TransfoXLLMHeadModel(TransfoXLPreTrainedModel):
             return self.out_layer
         else:
             return self.crit.out_layers[-1]
+
+    def prepare_inputs_for_generation(self, input_ids, **model_kwargs):
+        inputs = {"input_ids": input_ids}
+
+        # if past is defined in model kwargs then use it for faster decoding
+        if 'past' in model_kwargs and model_kwargs['past']:
+            inputs['mems'] = model_kwargs['past']
+
+        return inputs