Fix classification script: enable dynamic padding with truncation
# What does this PR do?

To fix the issue (below) in the run_glue.pl script, tokenizer's `max_length` value is now assigned directly with `max_seq_length` argument. Now it is possible to truncate the sequence and use dynamic padding. By default `max_length` is 128, which means truncation to 128 tokens. To disable truncation, set `max_length = None`.

Fixes #9551 

## 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/)? https://github.com/huggingface/transformers/issues/9551#issue-784679542
- [ ] 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?
@sgugger 