[Copy to main] fix(ssm): handle alignment padding in GDN packed seq + CP

[svcnvidia-nemo-ci]

🤖 This PR was auto-generated by Claude via the /claude copy command.

Cherry-picked from #4230.

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What does this PR do ?

Fix shape mismatch in GDN packed sequence + CP > 1 when alignment padding is present.

Summary

When cu_seqlens_q_padded is not provided, GDN falls back to cu_seqlens_q (actual token boundaries) for the per-sequence all-to-all unpack. Alignment padding tokens are lost, making the output shorter than hidden_states, crashing at the residual connection.

Fix: Extract cu_seqlens resolution into _resolve_cu_seqlens():

• cu_seqlens_padded available → use it (unchanged behavior)
• CP = 1 → use actual (no all-to-all, no issue)
• Single sequence + padding → auto-extend to cover trailing padding
• Multi sequence + padding → raise ValueError requesting cu_seqlens_padded

Test results

Unit tests: 5/5 passed

(2× H100, CP=2):

• Single seq + padding + no cu_seqlens_padded → auto-extend, output shape correct
• Multi seq + padding + no cu_seqlens_padded → ValueError raised correctly
• With cu_seqlens_padded set → standard path works
• No padding → passes through unchanged

Fixes #4194

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[yuzhongw-nvidia]

This fix could be involved in #2645. Close this one.