[ET-VK][qconv] Add software fallback for dotPacked4x8AccSatEXT in q8ta_ shaders

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][testing] Add ETVK_FORCE_NO_EXTENSIONS debug flag #17705
• -> [ET-VK][qconv] Add software fallback for dotPacked4x8AccSatEXT in q8ta_ shaders #17704
• [ET-VK][qconv] Read weight buffer as int in pack_q8_conv2d_weights shader #17703

Devices that lack VK_KHR_shader_integer_dot_product (older GPUs, emulators)
currently fail with ShaderNotSupportedError when running int8-quantized
conv2d/linear because the q8ta_ shaders unconditionally require
GL_EXT_integer_dot_product. This adds fallback SPIR-V variants that use a
pure-GLSL software implementation so those devices can still execute the
operators at a performance cost.

Approach: compile-time macro USE_INT8_DOT_PRODUCT_EXT selects the
implementation. Each affected YAML file gains a *_fallback shader variant
compiled with USE_INT8_DOT_PRODUCT_EXT=0. At C++ dispatch time,
adapter_ptr()->supports_int8_dot_product() picks the matching variant.

Changes:

• common.glslh: add dotPacked4x8Acc_fallback() and dotPacked4x8AccSat()
  dispatch macro
• linear_fp_output_tile_int8_int8_compute.glslh: guard extension + use macro
• q8ta_conv2d/pw/linear/linear_gemv .glsl: inject USE_INT8_DOT_PRODUCT_EXT
  template define, guard extension, replace direct EXT calls with macro
• q8ta_conv2d/pw/linear/linear_gemv .yaml: add USE_INT8_DOT_PRODUCT_EXT
  parameter and *_fallback shader variants
• Q8taConv2d/PW/Linear/LinearGemv .cpp: call supports_int8_dot_product() to
  select hardware vs. fallback variant at runtime

Differential Revision: D94314256

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job

As of commit 616c756 with merge base 63f9724 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
• pull / android / run-emulator (gh)
  The process '/opt/android/sdk/platform-tools/adb' failed with exit code 224
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 429afbe58116d55f0680b06bd104bb667a1bf1bd53d4dfb14c69538a816fe93f /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-buck / macos / macos-job (gh)
  ##[error]The operation was canceled.

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