[Common] Reduced padding kernel compilation time

[Oleg-Goncharov]

Description

This PR reduces the compilation time of padding.cu from approximately 600 seconds to 15 seconds by lowering the outer-loop unroll factor.

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:

• Changed the outer #pragma unroll directive from 8 to 4.
• Reduced compile-time overhead in the padding kernel.

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 padding.cu compilation time from ~600s to ~15s by changing #pragma unroll (full unroll) to #pragma unroll 4 in the outer loop of both multi_padding_kernel and multi_unpadding_kernel. Since n_iterations is a compile-time constant of 8 (THREADS_PER_WARP / n_warps_per_tile = 32 / 4), the original bare directive caused the compiler to fully unroll all 8 iterations across every template instantiation in TRANSFORMER_ENGINE_TYPE_SWITCH_ALL, producing massive code and very long compile times. Halving the unroll factor to 4 is a sensible trade-off with negligible runtime impact on these memory-bandwidth-bound kernels.

Confidence Score: 5/5

Safe to merge — minimal two-line change with no correctness risk and a large compile-time benefit.

n_iterations is a compile-time constant of 8, so #pragma unroll 4 produces two clean groups of 4 iterations with correct semantics. The kernels are memory-bandwidth-bound so the reduced unroll factor has negligible runtime impact. No logic, data, or API changes were made.

No files require special attention.

Important Files Changed

Filename	Overview
transformer_engine/common/util/padding.cu	Changed #pragma unroll to #pragma unroll 4 in both multi_padding_kernel and multi_unpadding_kernel; reduces compile time ~40x with minimal expected runtime impact.

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