Update embedding_bag.py for save_model in 'tf' mode

[AlexanderLavelle]

self.input_length is not defined, which affects the pickling / unpickling of this layer in tf model mode.

Description

Brief Description of the PR:

Of the two options of adding input_length or removing it entirely, it seems more elegant to remove it. The attribute input_length was likely ported from TF Embedding Layer, where it might be necessary; however, in EmbeddingBag there does not appear to be such requirements.

The alternative way to solve the issue is to take input_length as an int argument set to None but this seems likely to be more confusing and would additionally require a modification to the documentation

Fixes # (issue)
2803

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
  • 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:

• This is a simple modification of the removal of a line which is unused. If you look in VS Code you would see that the original is undefined. This layer now works as intended in save_model and load_model.

[bot-of-gabrieldemarmiesse]

@Rocketknight1

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.

[Rocketknight1]

Sorry for the delay, and sorry for leaving this issue in! The change seems good to me.