Fused dLN + add in backwards pass

[CarlosGomes98]

What does this PR do ?

dev branch: #3383
Fuse the backwards pass of the residual fork + LN pattern in transformer_layer.py.

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[Phlip79]

/ok to test 9125dc1

[Phlip79]

/ok to test 0440941

[CarlosGomes98]

Refactored to guard the has_residual flag behind a check for TENorm. I chose this approach as a similar pattern exists in the MLP fusion. However, I wonder if these decisions instead belong at the submodule picking level, so as not to keep cluttering the transformer_layer.

[ko3n1g]

/ok to test aa59424

[CarlosGomes98]

/ok to test 3fcb878

[ericharper]

/claude review

[ericharper]

/claude review

[ericharper]

/ok to test bd7395a

[Phlip79]

/ok to test 3d5501a

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22835292757