Inference: Do not route pad/dummy tokens to any expert

[sidsingh-nvidia]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do ?

Prevents routing of padding / dummy tokens to any of the routed experts. For small decodes, these can end up unnecessarily activating experts. This PR makes it such that only real tokens activate experts. This can help us move away from recording way too many fine grained cuda-graphs.

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[sidsingh-nvidia]

/claude review

[sidsingh-nvidia]

/claude review

[claude]

LGTM

[kvareddy]

@santhnm2 can you please take a look at this MR?

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[Phlip79]

@sidsingh-nvidia please fix DCO error

[sidsingh-nvidia]

@Phlip79 the suggested rebase commands always end up messing the commit-tree. I am having to do this manually.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/29313280742

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/29319016188