Adds license information for default and distilbert models
# What does this PR do?

Adds license information for default and `distilbert*` models. A follow up to #7668.

- Apache 2.0 for `distilbert*` based on https://github.com/huggingface/transformers/issues/3357#issuecomment-614856396
- MIT for `facebook/bart-large-mnli` based on https://github.com/huggingface/transformers/issues/7668#issuecomment-706064737

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
- [ ] 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. #7668 
- [ ] 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?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

Model Cards: @julien-c
Thanks!<|||||>Thanks @julien-c!