Change asserts in src/transformers/models/xlnet/ to raise ValueError
# What does this PR do?


Replaces asserts in:
src/transformers/models/xlnet/configuration_xlnet.py and src/transformers/models/xlnet/modeling_tf_xlnet.py  
to a ValueError exception.

Contributes towards fixing issue #12789


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss 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 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?

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@sgugger 
