Miscellaneous MTP inference fixes

[santhnm2]

What does this PR do ?

Makes the following fixes for MTP:

• Uses decode attention kernels instead of prefill kernels by gating on is_decode_only instead of max_seqlen_q > 1
• Fixes bug with double-counting accepted tokens when comparing against max_sequence_length to determine request termination
• Ensures that log probs / top n log probs are trimmed the same way as the tokens

Also updates the dummy rank initialization to add requests AFTER the expert parallel synchronization rather than before.

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[santhnm2]

/ok to test f499e39

[santhnm2]

/ok to test 8dac718

[santhnm2]

/ok to test a39e0cb

[santhnm2]

/ok to test b3ae69a

[santhnm2]

/ok to test 18e171d

[santhnm2]

/ok to test 67c5544

[santhnm2]

/ok to test 83d3d18

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24172283439