[megatron] fix: avoid mindspeed rope and attention synchronization

[Shangwei-Li]

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By default cu_seqlens is generated on device, and causing synchronization during rope and attention when using Mindspeed backend. This pr fix it by move cu_seqlens to cpu before forward.

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[gemini-code-assist]

Code Review

This pull request addresses a synchronization issue with the Mindspeed backend by moving cu_seqlens tensors to the CPU. The change is functionally correct and resolves the described problem. I've included a minor refactoring suggestion to improve code clarity and avoid redundant operations.

[gemini-code-assist]

To improve readability and avoid redundant calls, it's better to move cu_seqlens_padded to the CPU once and store it in a variable. This variable can then be used for all the cu_seqlens parameters.

    cu_seqlens_padded_cpu = cu_seqlens_padded.cpu()
    packed_seq_params = PackedSeqParams(
        qkv_format="thd",
        cu_seqlens_q=cu_seqlens_padded_cpu,
        max_seqlen_q=max_seqlen_in_batch,
        cu_seqlens_kv=cu_seqlens_padded_cpu,
        max_seqlen_kv=max_seqlen_in_batch,
        cu_seqlens_q_padded=cu_seqlens_padded_cpu,
        cu_seqlens_kv_padded=cu_seqlens_padded_cpu,
    )