Add code formatter yapf to the tests and pre-commit

[akihironitta]

What does this PR do?

Adds yapf https://github.com/google/yapf to CI and pre-commit hooks and is part of #482.

TODO

• add yapf to pre-commit hooks
• add yapf to GitHub Actions
• update flake8 and isort configs to avoid conflicts with yapf
  • As far as I looked through some issues and PRs, we don't need to change flake8/isort configs.
• Apply yapf to other files than pl_bolts/ and tests/.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #509 (3452880) into master (c997f85) will decrease coverage by 0.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   79.58%   79.07%   -0.51%     
==========================================
  Files         102      102              
  Lines        5921     5921              
==========================================
- Hits         4712     4682      -30     
- Misses       1209     1239      +30     

Flag	Coverage Δ
cpu	25.65% <ø> (ø)
pytest	25.65% <ø> (ø)
unittests	78.58% <ø> (-0.51%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pl_bolts/datasets/cifar10_dataset.py	71.73% <0.00%> (-26.09%)	⬇️
pl_bolts/datasets/base_dataset.py	81.81% <0.00%> (-13.64%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c997f85...3452880. Read the comment docs.

[akihironitta]

Applied yapf in #510, and it looks neat.