Triton RMSNorm Optimizations

[Micky774]

Description

Optimizes the Triton RMSNorm forward and backward kernels and adds an LDS-tiled FP8 transpose path. Measured 10%-50% improvements across a representative shape sweep for bf16 w/ no quantization or FP8 quant, and improvements of 3x-8x on FP8 Transpose outputs.

Benchmarks generated by this script.

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:

• Loop-invariant hoisting.
  • Fwd non-blocked path: gamma load + ZERO_CENTERED_GAMMA adjustment + 1/n_cols hoisted outside the persistent row loop.
  • Bwd non-blocked path: same gamma hoist; inv_n_cols hoisted.
  • Bwd both paths: per-row c_scalar = nf*nf*grad_sum*inv_n_cols computed once before the dx/dg loop; dx expression refactored to nf * (dz*g - c*x) (saves one multiply per element).
• Autotune wiring for bwd kernels. _rmsnorm_bwd_triton and _rmsnorm_bwd_dg_reduce_triton now follow the impl + autotune-wrapper dispatch pattern already used by the fwd kernel. te_rmsnorm_bwd_triton takes an autotune: bool = True kwarg; when off it uses the previously-hardcoded num_warps=8 + fixed BLOCK_SIZE_M/N=128/64 reduce tile.
• External LDS-tiled FP8 transpose kernel. New _fp8_transpose_2d_impl (+ autotune wrapper) replaces the in-kernel out_transpose_ptr + cols * stride + row_idx strided byte stores that were uncoalesced (one byte per thread to a different cache line). The new kernel does a coalesced (BLOCK_M, BLOCK_N) read, tl.trans() for LDS-staged transpose, then coalesced strided write.

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

[alextmagro]

LGTM!

[aris134]

LGTM!

[ipanfilo]

It is not used in code

[wenchenvincent]

@ipanfilo Could you check if the comments has been addressed?

[Micky774]

Updated!

[wenchenvincent]

@alextmagro @aris134 I saw you had approved the PR. For the inline comments, let's also resolve conversation if the comments has been addressed.