[Common] Reduce default cfg's SMEM usage for mhc triton kernel

[kainzhong]

Description

Somehow with triton 3.8 it allocates more SMEM for my triton kernels than previous triton 3.6, which causes the mhc tests to fail due to OOM. Add stage=2 to reduce the default config's SMEM usage and let the CI pass for now

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Make the mhc proj kernel's default config (the first one) use a smaller stage number to reduce SMEM usage

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[greptile-apps]

Greptile Summary

This PR reduces shared-memory usage in the MHC Triton projection kernel by adding a two-stage configuration.

• Makes num_stages=2 the first configuration used when Triton autotuning is disabled.
• Adds the two-stage option to the candidates considered when autotuning is enabled.

Confidence Score: 5/5

The PR appears safe to merge, with the lower-stage configuration covered by the existing MHC correctness-test path.

The change only adjusts the Triton pipeline-depth candidates, and the forced default configuration is exercised across representative MHC projection shapes and data types without violating any identified kernel constraint.

Important Files Changed

Filename	Overview
transformer_engine/common/triton/mhc.py	Expands the forward projection kernel’s stage candidates from [3, 4] to [2, 3, 4]; no actionable correctness issue was identified.

_{Reviews (1): Last reviewed commit: "[Common] Reduce default cfg's SMEM usage..." | Re-trigger Greptile}