Fix: resume issues with resuming in combined streaming dataset in dataloader

[bhimrazy]

Before submitting

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

How does this PR impact the user?

Currently, users experience issues when attempting to resume a combined streaming dataset with the streaming dataloader, as saving and restoring checkpoints doesn’t work as expected. This PR addresses the root cause of the error, enabling successful checkpoint resuming of the dataloader, ensuring smoother and more reliable training workflows.

What does this PR do?

Fixes #331.

• Fixes IndexError when loading dataloader state before any iteration.
• Enables resuming dataloader states for combined datasets (non-weighted).

This pr is the extension of #362

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

A lot , actually! 🙃
The PR (#362) had been pending since last September, but now, the underlying issue has finally been resolved with #449.

[bhimrazy]

🤞🫣

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (a8fc6a8) to head (bb16117).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #507   +/-   ##
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  Coverage    79%    79%           
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  Files        39     39           
  Lines      5844   5848    +4     
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+ Hits       4591   4602   +11     
+ Misses     1253   1246    -7     

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