fixed BertForMultipleChoice model init and forward pass
the number of choices is not respected because you've hardcoded '1' into the classifier layer.  also `token_type_ids` and `attention_mask` will cause an error if `None` because `None` does not have a `view` method.
Indeed, it looks better.
Do you want to have a look and confirm @rodgzilla?<|||||>@thomwolf any word on this?<|||||>Oh yes sorry. Looking at it and reading Alec Radford's paper on GPT (section 3.3) again, I think @rodgzilla was actually right in the original implementation.

So I guess we should close this PR.

I still would have been happy to get @rodgzilla input on that.<|||||>Oh sorry, we should still keep the `token_type_ids` and `attention_mask` `NoneType` fixes.
These ones are correct!