GA to Lint + Format MegaBlocks

[eitanturok]

This PR lints and formats MegaBlocks. Specifically, we:

• Create a .pre-commit-config.yaml with all of the same hooks as composer except for pyright (will be done in a subsequent PR).
• Formatted + Linted all code in MegaBlocks by running the pre-commit hooks. Changes include
  • adding a license
  • sorting imports
  • removing unusued imports
  • following style conventions in code
  • following style conventions in docstring
  • lint all python code
  • lint all yaml code
• Added a Github Action to automatically perform code formatting + linting

Note: I made all the .sh files in the exp directory executable as per the check-executables-have-shebangs hook.

[eitanturok]

I addressed all of your comments @b-chu except for taking care of fixing the __init__.py files in MegaBlocks.

All the tests pass, so it seems like this isn't such a bad issue...

[b-chu]

Fix the init files. Names should definitely match

[eitanturok]

I also added a contributing guide, style guide, and Pull_REQUEST template, all initially taken from composer.

[mvpatel2000]

can you change license to Databricks? Otherwise LGTM, would also like @b-chu to approve since large PR

[b-chu]

Fix all your file headers

[eitanturok]

@mihir-db I changed the license.

[b-chu]

Thanks for the PR!