fixing GPT2 double head model and updating the torch version tests

pytorch_transformers/modeling_utils.py

@@ -679,7 +679,7 @@ class SequenceSummary(nn.Module):
             self.last_dropout = nn.Dropout(config.summary_last_dropout)
 
     def forward(self, hidden_states, cls_index=None):
-        """ hidden_states: float Tensor in shape [bsz, seq_len, hidden_size], the hidden-states of the last layer.
+        """ hidden_states: float Tensor in shape [bsz, ..., seq_len, hidden_size], the hidden-states of the last layer.
             cls_index: [optional] position of the classification token if summary_type == 'cls_index',
                 shape (bsz,) or more generally (bsz, ...) where ... are optional leading dimensions of hidden_states.
                 if summary_type == 'cls_index' and cls_index is None: