Trainer accepts iterable datasets
Fixes #5990 
Follows PR #5995 (got closed for being stale...)

## 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?
- [X] 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.
- [X] 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).
- [X] Did you write any new necessary tests? 


## Who can review?
@sgugger @LysandreJik 


Hi there, thanks for the PR! Unfortunately it's not based on current master, so we can't review properly as it suggests changes from code that does not exist anymore.<|||||>Some merging work to be done.