Fix documented shape

[janEbert]

What does this PR do ?

The output shape is different when self.context_parallel_lm != 1. This is now properly documented.

@duncanriach I had trouble reasoning about #3476 from just the docstrings; then I saw the previously not considered conditional transposition. :)

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[yashaswikarnati]

@parthmannan FYI

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/24464624994