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

• [X] New Metric and the changes conform to the metric contribution guidelines

Checklist:

• [X] I've properly formatted my code according to the guidelines
  • [X] By running pre-commit hooks
• [ ] This PR addresses an already submitted issue for TensorFlow Addons
• [X] I have made corresponding changes to the documentation
• [X] 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.

[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: https://github.com/tensorflow/addons/pull/2741/commits/b4977f41b44263406c84f28effb64428a2224200

[bhack]

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