Code parrot minor fixes/niceties
# What does this PR do?

Hi there, really excited by HF expanding into Deep Learning for Software Engineering! This PR just adds a few fixes/niceties I found while testing out the scripts. Specifically:
1. Move the training code to a main function to allow using TPUs (TPU testing still needs to be done, but GPU training still works) with accelerate since it expects some entry function
2. Add some additional flags to human eval script for specifying which device to run the `text-generation` pipeline on and to specify how many tasks to evaluate against as it was previously hard-coded to 4
3. Update documentation to have the correct batch size flag for the validation set (was `eval_batch_size`, but should be `valid_batch_size`), add command for human eval script to use flag `--HF_ALLOW_CODE_EVAL="1"` since the program will throw an exception after generating all the samples, and add a note to make sure to install git-lfs for initializing/training the model.
4. Fix one of the requirements (`huggingface-hub`) to be `0.1.0` so that it does not conflict with the other libraries.

Since I couldn't get TPU training to work even with moving everything into the main function, I can separate out that commit if y'all would prefer into a separate WIP PR while I work on it. Just lemme know :nerd_face:!

You can verify my changes with this colab! https://colab.research.google.com/drive/1Tn0wsqqNbAEpxDn2AFs13Ssqwb0r3o-x?usp=sharing

## Before submitting
- [x] 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?
- [ ] 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.
- [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).
- [ ] Did you write any new necessary tests?


## Who can review?

- research_projects/codeparrot: @lvwerra
Alrighty @lvwerra, I think this PR should be (:crossed_fingers:) good to go :rocket:! Lemme know if anything else needs to be done!