Add Kendall's Tau-c, Spearman's Rank, Pearson's Correlation metrics

[nicolaspi]

Description

Add Kendall's Tau-c, Spearman's Rank, Pearson's Correlation metrics.

Type of change

• New Metric and the changes conform to the metric contribution guidelines

Checklist:

• I've properly formatted my code according to the guidelines
  • By running pre-commit hooks
• This PR addresses an already submitted issue for TensorFlow Addons
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• This PR contains modifications to C++ custom-ops

How Has This Been Tested?

The tests follows the contribution guidelines for metrics (https://github.com/tensorflow/addons/blob/master/tensorflow_addons/metrics/README.md#contribution-guidelines).
The metrics are tested against the scipy implementation counterpart.

[bot-of-gabrieldemarmiesse]

@sorensenjs

You are owner of some files modified in this pull request.
Would you kindly review the changes whenever you have the time to?
Thank you very much.

[sorensenjs]

This looks like a good change, but it's very hard to see the diffs - is it possible to either branch the file so that it shows diffs, or separate the renaming of the file from the other edits.

I'm not a frequent github user - if there's an easy way for me to see the diffs that would be helpful.

[nicolaspi]

There is no change per se, the original code remains the same modulo some refactoring. KendallsTau has been renamed KendallsTauB (to differenciate from KendallsTauC).

I have factored update_state from KendallsTau into a base class CorrelationBase with the abstract method result. For each metric, there is a concrete class that implements result with the corresponding metric's logic.

[nicolaspi]

@sorensenjs I changed the commit history to highlight the differences, this is the best I could come up with: b4977f4

[sorensenjs]

Thanks for the clarification. This is a nice generalization.

[bhack]

@nicolaspi Can you PR the codeowners file to match this refactoring?