add checkpoint_max_to_keep to train_loop

[FirdowsRahaman]

Description

:memo: Summary of the change.

• Sometimes we need more checkpoints to evaluate the trained model. If the user can define that on command line it'll help them to test and get a better model.
• No dependencies that are required for this change.

Type of change

• [ ] Other (Add checkpoint_max_to_keep argument to train_loop function)

Tests

:memo: Please describe the tests that you ran to verify your changes.

• Provide instructions so we can reproduce.
• Please also list any relevant details for your test configuration.

Test Configuration:

Checklist

• [ ] I have signed the Contributor License Agreement.
• [ ] I have read guidelines for pull request.
• [ ] My code follows the coding guidelines.
• [ ] I have performed a self code review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have made corresponding changes to the documentation.
• [ ] My changes generate no new warnings.
• [ ] I have added tests that prove my fix is effective or that my feature works.

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