StopIteration error in RobertaForMultipleChoice
Hello,
I am trying to execute the line below for my `RobertaForMultipleChoice` model:
```python
# retrieve the resulting mc_loss
mc_loss = model(input_ids = input_ids, attention_mask = attention_mask, labels =  mc_labels)[0]
```
but this generates the following error:

```python
Traceback (most recent call last):
  File "STAT946_final_project_code_v4.py", line 623, in <module>
    success_rate_list_diag_normal = main_function_diag_normal('/home/ec2-user/test.txt', 'test_ans_num.txt', num_iter, log_interval)
  File "STAT946_final_project_code_v4.py", line 414, in main_function_diag_normal
    best_model_RobertaForMultipleChoice_diag_normal = train_loop(model_RobertaForMultipleChoice, tokenizer, optimizer_1, scheduler_1, log_interval, svi_diag_normal, guide_diag_normal, best_model_RobertaForMultipleChoice_diag_normal)
  File "STAT946_final_project_code_v4.py", line 341, in train_loop
    optimizer, scheduler, log_interval, svi, guide, epoch)
  File "STAT946_final_project_code_v4.py", line 236, in train_mc_head
    mc_loss = model(input_ids = input_ids, attention_mask = attention_mask, labels =  mc_labels)[0]
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/pyro/nn/module.py", line 413, in __call__
    return super().__call__(*args, **kwargs)
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 722, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/transformers/modeling_roberta.py", line 441, in forward
    output_hidden_states=output_hidden_states,
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/pyro/nn/module.py", line 413, in __call__
    return super().__call__(*args, **kwargs)
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 722, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/transformers/modeling_bert.py", line 732, in forward
    extended_attention_mask: torch.Tensor = self.get_extended_attention_mask(attention_mask, input_shape, device)
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/transformers/modeling_utils.py", line 228, in get_extended_attention_mask
    extended_attention_mask = extended_attention_mask.to(dtype=self.dtype)  # fp16 compatibility
  File "/home/ec2-user/anaconda3/lib/python3.7/site-packages/transformers/modeling_utils.py", line 159, in dtype
    first_tuple = next(gen)
StopIteration
```

How can I get around this type of error? Thank you,