[Example] Fix the argument name mismatch in the distillation example
Author: jayparks
Pull request: yes
Labels: name: wontfix

# What does this PR do?

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This past [PR](https://github.com/huggingface/transformers/pull/6315) replaced the argument name `n_gpu` of the distillation example with `gpus`. This causes a crash in running the example since the rest of the example code still uses the old argument name (`n_gpu`). 

This PR solves the issue and get the distillation example to run.



## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      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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Comments:
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