[Fix] Compatible with TF2.15 register_checkpoint_saver when multiple TFRA Embedding in one trackables.

[MoFHeka]

Description

In my previous tests, there was only one TFRA Embedding object passed into the trackables in the save loading function, even though there were multiple TFRA Embedding objects in the model. However, we have tested that under certain composition conditions, the trackables may contain multiple TFRA Embedding, so the group of multiple loading functions should be returned.

Type of change

• [x] Bug fix
• [ ] New Tutorial
• [ ] Updated or additional documentation
• [x] Additional Testing
• [ ] New Feature

Checklist:

• [x] I've properly formatted my code according to the guidelines
  • [x] By running yapf
  • [x] By running clang-format
• [ ] This PR addresses an already submitted issue for TensorFlow Recommenders-Addons
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works

How Has This Been Tested?

Run the test.