Simplify exclude_from_weight_decay implementation

[lgeiger]

Description

This PR changes the handling of exclude_from_weight_decay to ensure that the regex is only evaluated once per step. Currently it gets called within a distributed merge_call which might lead to problems.
I implemented this when trying to debug a performance regression which turned out to be unrelated to this, but I think this still is a nice cleanup to have since it consolidates the handling of decay_var_list and exclude_from_weight_decay.

Type of change

• Bug fix
• New Tutorial
• Updated or additional documentation
• Additional Testing
• New Activation and the changes conform to the activation contribution guidelines
• New Callback and the changes conform to the callback contribution guidelines
• New Image addition and the changes conform to the image op contribution guidelines
• New Layer and the changes conform to the layer contribution guidelines
• New Loss and the changes conform to the loss contribution guidelines
• New Metric and the changes conform to the metric contribution guidelines
• New Optimizer and the changes conform to the optimizer contribution guidelines
• New RNN Cell and the changes conform to the rnn contribution guidelines
• New Seq2seq addition and the changes conform to the seq2seq contribution guidelines
• New Text addition and the changes conform to the text op contribution guidelines

Checklist:

• I've properly formatted my code according to the guidelines
  • By running Black + Flake8
  • 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?

I adapted the unittests that where directly relying on the internal functions that had been modified.

[bot-of-gabrieldemarmiesse]

@PhilJd

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.