[ExecuTorch][Vulkan] Add et_vk.linear_q4gsw_dw kernel (dense fp32 weight-grad GEMM)

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][Vulkan] Tests for et_vk.q4gsw_requant #20946
• [ExecuTorch][Vulkan] Add et_vk.q4gsw_requant kernel (STE re-quant to W_4X8) #20945
• [ExecuTorch][Vulkan] Tests for et_vk.linear_q4gsw_backward #20944
• [ExecuTorch][Vulkan] Add et_vk.linear_q4gsw_backward kernel (4-bit input-grad) #20943
• [ExecuTorch][Vulkan] Tests for et_vk.fused_ce #20942
• [ExecuTorch][Vulkan] Add et_vk.fused_ce kernels (fused cross-entropy loss + dlogits) #20941
• [ExecuTorch][Vulkan] Tests for et_vk.linear_q4gsw_dw #20940
• -> [ExecuTorch][Vulkan] Add et_vk.linear_q4gsw_dw kernel (dense fp32 weight-grad GEMM) #20939
• [ExecuTorch][Vulkan] Tests for et_vk.adamw_step #20938
• [ExecuTorch][Vulkan] Add et_vk.adamw_step kernel (on-GPU AdamW optimizer step) #20937
• [ExecuTorch][WebGPU] Tests for the adamw WebGPU ops #20936
• [ExecuTorch][WebGPU] Add et_vk.adamw_step (on-GPU AdamW optimizer step) to the WebGPU backend #20935
• [ExecuTorch][WebGPU] Tests for the fused_ce WebGPU ops #20934
• [ExecuTorch][WebGPU] Add et_vk.fused_ce (fused cross-entropy) to the WebGPU backend #20933
• [ExecuTorch][WebGPU] Tests for the q4gsw_train WebGPU ops #20932
• [ExecuTorch][WebGPU] Add q4gsw training kernels (backward, weight-grad, requant) to the WebGPU backend #20931
• [ExecuTorch][WebGPU] Tests for the shape_copy WebGPU ops #20930
• [ExecuTorch][WebGPU] Add shape/copy ops (dim_order, expand_copy, fill) to the WebGPU backend #20929
• [ExecuTorch][WebGPU] Tests for the gather WebGPU ops #20928
• [ExecuTorch][WebGPU] Add gather and embedding ops to the WebGPU backend #20927
• [ExecuTorch][WebGPU] Tests for the select_bool WebGPU ops #20926
• [ExecuTorch][WebGPU] Add select/boolean ops (where, scalar compares, logical_not) to the WebGPU backend #20925
• [ExecuTorch][WebGPU] Tests for the reduce WebGPU ops #20924
• [ExecuTorch][WebGPU] Add reduce op (sum/mean) to the WebGPU backend #20923
• [ExecuTorch][WebGPU] Tests for the binary WebGPU ops #20922
• [ExecuTorch][WebGPU] Add elementwise binary ops (div, sub) to the WebGPU backend #20921
• [ExecuTorch][WebGPU] Tests for the softmax WebGPU ops #20920
• [ExecuTorch][WebGPU] Add softmax ops (_softmax, _log_softmax) to the WebGPU backend #20919
• [ExecuTorch][WebGPU] Tests for the matmul WebGPU ops #20918
• [ExecuTorch][WebGPU] Add fp32 matmul ops (mm, bmm, linear) to the WebGPU backend #20917

Vulkan GLSL kernel + handler for the training custom op et_vk.linear_q4gsw_dw — a dense fp32 GEMM dW[N,K] = d_out^T @ x (contracts over M; NOT quantized despite the name). Mirrors impl/Matmul.cpp add_matmul_tiled_node structure + sizes_ubo for dynamic shapes; the M-contraction tile math is transcribed from the golden-verified reference. AOT registration already exists under the shared Vulkan partitioner.
@exported-using-ghexport

Differential Revision: D111761779

Differential Revision: D111761779

[pytorch-bot]

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❌ 2 New Failures

As of commit bfe0a6d with merge base 7013c8d ():

NEW FAILURES - The following jobs have failed:

• pull / test-vulkan-operators-linux / linux-job (gh)
  /pytorch/executorch/backends/vulkan/test/op_tests/adamw_step_test.cpp:65:3: error: no member named 'manual_seed' in namespace 'torch'; did you mean 'at::manual_seed'?
• Test Vulkan Backend (specialized runners) / test-vulkan-nvidia / linux-job (gh)
  /pytorch/executorch/backends/vulkan/test/op_tests/adamw_step_test.cpp:65:10: error: ‘manual_seed’ is not a member of ‘torch’; did you mean ‘at::manual_seed’?

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