[NPU] Add optimized NPU mhc

[lowdy1]

Add Ascend NPU Triton kernels for the three mHC sub-operators:

• Fused matmul + RMS normalization (forward/backward)
• Sinkhorn routing with split pre/post/residual coefficients (forward/backward)
• Pre-aggregate weighted sum (forward/backward)
• Post + residual mixing (forward/backward)

NPU optimizations applied:

• Unified UB tiling via compute_default_tiling_strategy for matrix
• Persistent grid-stride loops (tl.range + num_programs)
• Adaptive BLOCK_N/BLOCK_M for core utilisation at small seq_len
• Fused backward coefficient assembly kernel

Hardware Type: Atlas 800I A2

• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence