fix nn.DataParallel compatibility with PyTorch 1.5
The same type of errors as in https://github.com/huggingface/transformers/pull/4300

# What does this PR do?
DataParallel replicate has a known issue in PyTorch 1.5: https://github.com/pytorch/pytorch/issues/40457

A similar PR proposes a work around by removing the `next(self.parameters().dtype)`: https://github.com/huggingface/transformers/pull/4300/files/7eef4f5a7575e05e822f8ef45d7f473a102671aa

I did the same in LXMERT


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), 
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to the it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests? 


## Who can review?

@patrickvonplaten
 @julien-c 
(tagging @eltoto1219 for information)