Add unittests for AnyPrecisionOptimizer

[rohan-varma]

What does this PR do? Please describe:
Adds basic unittest for AnyPrecisionOptimizer to ensure it is equivalent to Adam under fp32 configuration. We should have unittests as the feature is being used by e.g. @stas00 and it would be good to provide correctness guarantees.

Fixes #57

Does your PR introduce any breaking changes? If yes, please list them:
List of all backwards-incompatible API changes.

Check list:

• Was this discussed and approved via a GitHub issue? (not for typos or docs)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, or minor internal changes)

[H-Huang]

lgtm