add API reference and tests

[ananyahjha93]

What does this PR do?

Adds tests for optimizers and API references.

Fixes # (issue)

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

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[codecov]

Codecov Report

Merging #680 (1b7c2c5) into master (8b27498) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #680   +/-   ##
=======================================
  Coverage   89.67%   89.67%           
=======================================
  Files         192      192           
  Lines        9905     9905           
=======================================
  Hits         8882     8882           
  Misses       1023     1023           

Flag	Coverage Δ
gpu	?
pytest	?
unittests	89.67% <ø> (ø)

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

Impacted Files	Coverage Δ
flash/core/optimizers/lamb.py	34.28% <ø> (ø)
flash/core/optimizers/lars.py	49.09% <ø> (ø)

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[ethanwharris]

LGTM 😃