Add pre-commit hook to remove trailing white spaces

[cgarciae]

What does this PR do?

• Adds the trailing-whitespace pre-commit hook.
• Removes trailing whitespaces from all files in the project.

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[codecov-commenter]

Codecov Report

Merging #2513 (442df07) into main (ad331b9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2513   +/-   ##
=======================================
  Coverage   79.09%   79.09%           
=======================================
  Files          49       49           
  Lines        5180     5180           
=======================================
  Hits         4097     4097           
  Misses       1083     1083           

Impacted Files	Coverage Δ
flax/core/lift.py	95.88% <ø> (ø)
flax/errors.py	87.20% <ø> (ø)
flax/linen/linear.py	97.50% <ø> (ø)
flax/linen/module.py	92.73% <ø> (ø)
flax/linen/summary.py	99.01% <ø> (ø)
flax/linen/transforms.py	93.90% <ø> (ø)
flax/training/checkpoints.py	61.47% <ø> (ø)
flax/training/early_stopping.py	100.00% <ø> (ø)
flax/training/lr_schedule.py	0.00% <ø> (ø)
flax/traverse_util.py	98.52% <ø> (ø)
... and 1 more

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[levskaya]

Looks great - you mentioned that the CI actually checks this somehow - but I don't see us modifying any CI configs - does github CI actually look at the .pre-commit-config.yaml file or something?

[cgarciae]

@levskaya we have this github action that checks that all pre-commit hooks as specified by pre-commit-config.yaml pass:

flax/.github/workflows/build.yml

Line 31 in ad331b9

- uses: pre-commit/action@v2.0.3

[levskaya]

Ah thanks! I was just blind.