[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 #17692
• -> [ET-VK][qconv] Add software fallback for dotPacked4x8AccSatEXT in q8ta_ shaders #17691
• [ET-VK][qconv] Read weight buffer as int in pack_q8_conv2d_weights shader #17690

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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❌ 2 New Failures

As of commit 5cc872f with merge base 63f9724 ():

NEW FAILURES - The following jobs have failed:

• Propose to merge ghstack orig PRs to main / Try to create a PR with ghstack /orig branch (gh)
  Process completed with exit code 1.
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

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