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* Replaced PyTorch's slow permutation ops with custom kernels, significantly improving performance (especially on GB200).
* Split kernel into general and specialized versions for num_channel <= 16384, significantly reducing memory accesses.
* Enabled float4-based vectorized memory access when pointer alignment and channel size allow, improving throughput.
* Added runtime dispatch logic for kernel specialization.

Small fix in metric computation

Optimize bwd kernel: incremental qdot_max and alpha/integral/etc

adding lineinfo to optional debug flags

Leverage the same qdotk_max "trick" for the backward kernel. This avoids 1 loop
and saves about 20% of performance.

fixing attention perf test attempt 1

Optimized CUDA kernels for S2 Attention (forward and backward)

Merge branch 'mr/bwd-channel-permute-experiments' of https://github.com/rietmann-nv/torch-harmonics into mr/bwd-channel-permute-experiments

Also: Made fwd kernel use modified memory layout with standard shape

Also match the gradient output to the input, in terms of memory layout

Detect memory layout (B,C,H,W) (stride for C should be 1, if not, fix it)

This ensures that the backwards kernel is fast

putting qy in shared is a little faster

Changing internal memory layout means we can leave code in standard shape and
only change layout external to kernel

Introduce new CUDA kernels, `s2_attention_bwd_dkvq_kernel_mbT` and
`s2_attention_kernel_mbT`, for more efficient computation of backward gradients
and forward attention respectively. These changes optimize memory access
patterns and employ coalesced operations by leveraging tensor transpositions.

Forward kernel written by Mauro Bisson
Backwards kernel written by Andrea Paris (aparis@ethz.ch) and Max Rietmann

Parallelization strategy computes 1 output per Warp, with threads computing the
dot-product in parallel. Because inputs are transposed to have channel dimension
last, the dot-product memory access pattern is perfectly coalesced, leading to
excellent performance. This is true across both forward and backward kernels.
Co-authored-by: Mauro Bisson <maurob@nvidia.com>
Co-authored-by: Max Rietmann <mrietmann@nvidia.com>
Co-authored-by: Andrea Paris <aparis@ethz.ch>

fixing bug in quadrature weights for full attention. Adding better unit tests for attention. Cleanup in the cuda code.