Add AdaBelief optimizer

[juntang-zhuang]

Description

Add AdaBelief in tensorflow_addons, which adapts stepsize according to "belief" in gradient, and achieves good generalization, fast convergence and training stability.

Reference: AdaBelief optimizer: adapting stepsizes by the belief in observed gradients (Juntang Zhuang et al. NeurIPS 2020).

Results: 77.25 (AdaBelief) 77.1 (SGD)

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?

By running a ResNet50 model from google/tensorflow_models benchmark

If you're adding a bugfix or new feature please describe the tests that you ran to verify your changes:
*

[AakashKumarNain]

@juntang-zhuang please format the code, and make sure your flake8 and black versions are same as in tf-addons

[AakashKumarNain]

Thank you for the contribution. This is a good addition to addons

[juntang-zhuang]

Thanks for your help! @AakashKumarNain