Beautifier losses doc

[WindQAQ]

Description

Beautifier losses doc. Our doc is very messy 😢.

• Encourage to use backticks to wrap the shape. https://www.tensorflow.org/community/contribute/docs_style#tensors
• For tf.*, one should wrap it with backticks so that the hyperlink can be generated. https://www.tensorflow.org/community/contribute/docs_style#links_in_markdown
• Encourage to use mathjax syntax for math expr. https://www.tensorflow.org/community/contribute/docs_style#math_in_markdown

https://www.tensorflow.org/addons/api_docs/python/tfa/losses

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?

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

[bot-of-gabrieldemarmiesse]

@SSaishruthi @wenmin-wu @AakashKumarNain @AndreasMadsen @lc0 @fsx950223 @RomainBrault @rahulunair

You are owners of some files modified in this pull request.
Would you kindly review the changes whenever you have the time to?
Thank you very much.

[bhack]

Did you use any tools?

[WindQAQ]

Did you use any tools?

No, I just use my eyes and build doc locally to check the result.

https://github.com/tensorflow/addons/tree/master/docs

[lc0]

Thanks for making the docs more beautiful! 🥳

[AakashKumarNain]

Thanks for doing this @WindQAQ This is really good

[seanpmorgan]

LGTM thanks @WindQAQ !