FSDP: Added fix + Regression test for incorrect params_group mapping in optimizer state sharding

[Shasha-17]

What does this PR do?

This PR fixes a correctness issue in FSDP optimizer state sharding where
param_groups were reconstructed using a global count derived from
param_id_map.values().

The previous logic relied on itertools.groupby, which only removes
adjacent duplicates and assumes ordered, contiguous IDs. This could
produce incorrect parameter mappings and silently corrupt optimizer
state for non-contiguous or partially unused parameter layouts.

What is changed?

• Compute unique, valid local parameter IDs from param_id_map
• Exclude None entries corresponding to unused parameters
• Assign consistent local parameter IDs to each param group
• Add a regression test covering non-contiguous and None mappings

Testing

• Added unit test in test_fsdp_optim_utils.py
• Ran pytest tests/nn/data_parallel/test_fsdp_optim_utils.py - failed before fix, passed after fix

Before submitting

• [Yes] Did you have fun?
  • Make sure you had fun coding 🙃
• [Yes] Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
  • N/A
• Did you make sure to update the docs?
  • N/A
• [Yes] Did you write any new necessary tests?
• Did you update the changelog? (if needed)
  • N/A

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.

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