Add initial GitHub workflows

[jon-tow]

This PR adds initial GitHub workflows for building, testing, and checking for code quality.

[jon-tow]

This relies on #42 for automatic flake8 dependency installation in the build.yml run.
Edit: Resolved with the latest merge.

[Dahoas]

This looks good to me though I'm really not familiar with setting up github actions.

Do you mind removing the torch-typing errors with the fix you noted in discord?

[jon-tow]

I've removed the "undefined name" check in the build workflow. CC @cat-state if they think we should add mypy in this PR or open a new one when we figure out how to properly set it up.

[Dahoas]

If we think mypy can be set up by end of the week I'm for adding to this PR. If not we can merge now.

[LouisCastricato]

Is this good to merge?

[cat-state]

I think let's do mypy on a subsequent one cus we'll have type errors and annotations to fix

[jon-tow]

Le'ts merge. I'm currently unfamiliar with mypy and Aman might be too busy to add it right now. We can move discussions/concerns to issue #40.