Fix assert for list of layers

[bhack]

Description

The assert don't accept list of layers ad in docs
Brief Description of the PR:
Accept list of layers
Fixes # (issue)
Fixes #2178

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
  • [ x] By running Black + Flake8
  • [ x] By running pre-commit hooks
• [ x] 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:
*

[bot-of-gabrieldemarmiesse]

@hyang0129

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.

[WindQAQ]

Hmm, seems that we are working on the same issue :-( @bhack Would you mind take a look at my PR? Thanks.
#2180