Add docstring for Correlation cost layer

[facaiy]

1. Hide correlation_cost function, and make it private.
2. Add document for CorrelationCost

For details please check the discussion in #207

[PyExtreme]

Hello @facaiy ,

I am new to open source but have good experience in ML. I am interested to contribute to addons and this seems to be a good starting task.
Could you please assign it to me?

Thanks

[facaiy]

Welcome, @PyExtreme :-)

[PyExtreme]

Hi @facaiy , @WindQAQ , I am facing some bazel related issue while running the tests. I am trying to solve it. I have followed the documentation as well.
It would be great if you could please provide me the commands list in sequence to set up the entire environment and run tests.

[PyExtreme]

Hi Everyone,I have fixed those bazel issues and ready to work..