black formatted codebase with pre-commit files

[anmolsjoshi]

Fixes #

Description:
Per discussion on slack, added black formatting to codebase.

• Updated CONTRIBUTING.md
• Updated .travis.yml to check for black in Linting stage
• Added .pre-commit-config.yaml to invoke flake8 for linting and black for autoformatting for pre-commit hooks
• Updated tox.ini to ignore certain flake8 errors that were referenced in black documentation
  • https://black.readthedocs.io/en/stable/the_black_code_style.html#slices
  • https://black.readthedocs.io/en/stable/the_black_code_style.html#line-breaks-binary-operators
• Code base was automatically formatted using black

Helpful links:

• Black documentation
• Black playground
• Pre-commit documentation

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[anmolsjoshi]

cc @justusschock @ykumards

[vfdev-5]

Thanks @anmolsjoshi ! Huge PR with 164 updated files, if CI passes it is OK for me.
I let Justus and Yogesh say their word too...

[justusschock]

I just got one small question: can't we use black also as a linter? This way it would be more concise than to format with black and to lint with flake and additional exceptions.

[ykumards]

Thanks @anmolsjoshi ! Huge PR with 164 updated files, if CI passes it is OK for me.
I let Justus and Yogesh say their word too...

love this!

[anmolsjoshi]

@justusschock black is primarily a code formatter. We can invoke it with a --check flag to check if all the files in the code base comply with, but it doesn't suggest what should be changed like flake8. Please see below.

black . --check
would reformat /home/joshi/ignite/tests/ignite/metrics/test_metric.py
would reformat /home/joshi/ignite/tests/ignite/metrics/test_accuracy.py
would reformat /home/joshi/ignite/tests/ignite/metrics/test_precision.py
would reformat /home/joshi/ignite/tests/ignite/metrics/test_recall.py
Oh no! 💥 💔 💥
165 files would be reformatted, 19 files would be left unchanged.

[vfdev-5]

LGTM! Thanks a lot @anmolsjoshi !