from_pretrained: convert DialoGPT format

DialoGPT checkpoints have "lm_head.decoder.weight" instead of "lm_head.weight". 

(see: https://www.reddit.com/r/MachineLearning/comments/dt5woy/p_dialogpt_state_of_the_art_conversational_model/f6vmwuy?utm_source=share&utm_medium=web2x)

transformers/modeling_utils.py

@@ -417,6 +417,8 @@ class PreTrainedModel(nn.Module):
                     new_key = key.replace('gamma', 'weight')
                 if 'beta' in key:
                     new_key = key.replace('beta', 'bias')
+                if key == 'lm_head.decoder.weight':
+                    new_key = 'lm_head.weight'
                 if new_key:
                     old_keys.append(key)
                     new_keys.append(new_key)