Add unit test for THD

[kunlunl]

What does this PR do ?

Add unit test to compare THD format against SBHD format.
main branch #3617

Test Strategy

1. Generate full (unsharded) data with deterministic seed on each rank.
2. Shard inputs for both SBHD and THD formats (zigzag CP, contiguous SP).
3. Forward pass through the same TransformerLayer.
4. Gather outputs back to full size (with gradient support).
5. Backward pass with format-specific grad_output handling.
6. Compare outputs and gradients with bitwise or similarity checks.

Check Levels

• bitwise_all: B=1, forward + backward bitwise (MockCoreAttention)
• bitwise_fwd: B>1, forward bitwise, backward similarity (MockCoreAttention,
  THD padded to max_len so total tokens match SBHD)
• similarity: All parallelism configs, real TE attention, similarity checks

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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• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
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[kunlunl]

/ok to test b44a3b5

[kunlunl]

/ok to test 027bd1a

[yanring]

LGTM—thanks for adding these great tests!

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22511910500