[Dev] Skip routed expert padding for graph-safe MoE

[zhongbozhu]

What does this PR do ?

This PR is a follow up fix to #3890, which introduced TE Fused group mlp layer for graph safe moe. However, padding itself as a stand-alone operation can also be graph-unsafe if the padding was implemented based on knowing problem shapes on host, which is the case for router padding and explicit Fp8Padding module.

HybridEp provides the ability of doing the padding fused with local permute, which is done on-device, so it's cuda graph safe. This PR checks whether padding has already been done so that we can safely skip the graph-unsafe padding, this will be used together with the paged stashing PR: #2690.

PR to main: TBD (waiting on #3971 to be merged to main)

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[buptzyb]

LGTM!

[yaox12]

/ok to test a10b0cc

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23793449235