[Main] Numerical fix for FC2 expert bias scales when using use_transformer_engine_op_fuser

[zhongbozhu]

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What does this PR do?

Triggered only for use_transformer_engine_op_fuser + routed expert has bias

The fused MoE MLP applies each token’s router probability before FC2:
fc2_input = router_probability * activation(fc1_output)
Previously, FC2 with bias produced:
output = router_probability * (fc2_weight @ activation) + fc2_bias
The correct result is:
output = router_probability * ((fc2_weight @ activation) + fc2_bias)

Numerical unit test

pytest -sv tests/unit_tests/transformer/moe/test_grouped_mlp.py::TestTEGroupedMLP::test_gpu_fused_path_scales_fc2_bias

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[zhongbozhu]

/ok to test a1913ae

[yashaswikarnati]

just a nit; looks like we dont check if fused impl is supported here but not in actual code above? is the current version of TE pinned is well above the min supported version ?

[zhongbozhu]

resolved

[zhongbozhu]

/ok to test 8ac399d

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You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/29956320233

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/29964343805

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/29968255282