Pull Request resolved: #17221

This commit adds a flexible memory layout implementation for quantized pointwise
(1x1) convolution in the ExecuTorch Vulkan backend. The key changes introduce a
new operator (etvk.q8ta_conv2d_pw) that can handle multiple int8 tensor memory
layouts, rather than being restricted to a single fixed layout.

Key Components Added

1. Two New GLSL Compute Shaders

- q8ta_conv2d_pw.glsl: The primary flexible-layout shader that uses
  BufferMetadata UBOs and layout specialization constants to support multiple
  memory layouts (kPackedInt8_4C1W, kPackedInt8_4W4C, kPackedInt8_4C). Uses
  scalar array indexing for output writes to handle different stride patterns.
- q8ta_conv2d_pw_4w4c_ref.glsl: A reference implementation specifically for 4W4C
  layout that uses simpler ivec4 indexing. Currently not enabled in production
  (gated by if (false) in C++).

Both shaders use:

- 4×8 output tiling (TILE_M=4 widths × TILE_N=8 channels per thread)
- dotPacked4x8AccSatEXT for efficient int8 dot products
- Texture2D for weight storage, buffers for input/output
- Per-channel weight quantization with symmetric int8 weights

2. C++ Operator Implementation (Q8taConv2dPW.cpp)

- prepack_quantized_conv2d_pw_weight(): Prepacks int8 weights into texture2D
  format optimized for the shader's access pattern
- add_q8ta_conv2d_pw_node(): Dispatches the flexible-layout shader with buffer
  metadata UBOs
- add_q8ta_conv2d_pw_4w4c_node(): Dispatches the 4W4C-specific reference shader
- q8ta_conv2d_pw(): High-level operator that handles argument parsing, weight
  prepacking, and kernel selection

3. Test Infrastructure Updates

- TestQ8taConv2d.cpp: Added test_q8ta_conv2d_pw() test operator that wraps
  quantize → conv2d_pw → dequantize for end-to-end testing
- test_q8ta_conv2d_pw.cpp: Comprehensive test suite with:
  - Multiple input sizes (3→32, 32→64, 64→96, 7→13, 40→80 channels, etc.)
  - Performance test cases (480→160, 48→22, 128→128, 576→64 channels)
  - Tests across 3 memory layouts: kPackedInt8_4C1W, kPackedInt8_4W4C,
    kPackedInt8_4C
  - Both texture and buffer storage types for floating-point tensors
  - Reference implementation comparison for correctness validation

Architecture

The shader handles layout flexibility via:

1. Layout specialization constants (outp_layout, inp_layout) passed from C++
2. BufferMetadata UBOs providing runtime strides for input/output tensors
3. compute_outp_buffer_idx() function that computes correct buffer indices based
   on layout
4. get_outer_packed_dim_block_size() from block_indexing.glslh to determine
   stride patterns
ghstack-source-id: 338638556
@exported-using-ghexport

Differential Revision: [D92307253](https://our.internmc.facebook.com/intern/diff/D92307253/)