Reformat codebase and add pre-commit

[zhijian-liu]

This PR reformats the codebase and integrates pre-commit for continuous integration. Contributors can first activate it locally by pre-commit install, which will then be triggered automatically at git commit to check, fix and reformat the commit.

[clee-ai]

Looks great! I'm curious, what's the advantage of doing it through pre-commit versus the old formatter workflow?

[zhijian-liu]

@CCInc, the advantages of pre-commit are mainly two-fold:

• It allows the contributors to perform the check locally before pushing the changes to the repo.
• It supports more than just reformatting the code: it also checks some type errors (see https://github.com/mit-han-lab/torchsparse/blob/dev/pre-commit/.pre-commit-config.yaml for more details).

[sandeepnmenon]

This is great!

[zhijian-liu]

@CCInc @sandeepnmenon, this PR is ready for review. @kentangSJTU, I've tested the correctness of this version on semantic segmentation. It would be great if you could test it on object detection as well.

[clee-ai]

Great! I love how clean the pytorch API is now.