Save and load task state to the checkpoint

[piyush-kansal]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• Did you read the contributor guideline?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

This PR adds support for the following:

• Saving task state/attributes to the checkpoint
• Repopulating the task state/attributes while loading the checkpoint
• The respective unit test cases have been added to validate the new functionality

Besides, this PR also removes an unused import.

PR review

CC @cbalioglu

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