Specify default values explicitly

[faizan-m]

Description

Brief Description of the PR: explicitly specify default values to prevent breakage from upcoming tf.function changes

Fixes # (issue)

Type of change

• [x ] 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:

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

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

[bhack]

@seanpmorgan Is the CI broken?