[ET-VK] Add fused q8ta_relu unary operator for int8x4 tensors

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][ez][qconv] Add auto-selection to prefer im2col for q8ta_conv2d #17568
• [ET-VK][q8ta] Fix addmm arg indexing in QuantizedLinearMatch #17567
• [ET-VK][q8ta] Add q8ta_linear_gemv op for batch-1 int8 linear #17566
• [ET-VK][q8ta] Add q8ta_linear operator for int8 quantized linear #17565
• [ET-VK][export] Update tensor representation sync logic to allow for flexibility in memory layouts #17564
• [ET-VK] Support different input layouts in q8ta_binary operator #17563
• [ET-VK][ez] Always partition batch norm as it will be fused #17508
• -> [ET-VK] Add fused q8ta_relu unary operator for int8x4 tensors #17507
• [ET-VK][qconv] Add apply_relu support to q8ta conv operators #17506
• [ET-VK][qconv] Pad weight_sums buffer to multiple-of-4 alignment #17505
• [ET-VK][qconv] Fix depthwise weight_sums sum dimension #17504

This adds a fused quantized unary operator (ReLU) that operates directly on int8x4 packed buffer tensors, avoiding the overhead of separate dequantize-relu-requantize dispatches.

The implementation follows the same pattern as q8ta_binary: a single GLSL compute shader dequantizes int8x4 blocks to float, applies the unary operation, and requantizes back to int8x4 in one dispatch. The shader uses the OPERATOR macro for parameterization so additional unary ops can be added as YAML variants without new shader code.

Components added:

• GLSL shader (q8ta_unary.glsl) and YAML config with relu variant
• C++ operator implementation (Q8taUnary.cpp/h) registering et_vk.q8ta_relu.default
• Export graph fusion pattern (quantized_unary.py) that detects dequant->relu->quant sequences and replaces them with the fused op
• Custom op definition (q8ta_relu in custom_ops_lib.py) for the export pipeline
• Test harness (TestQ8taUnary.cpp, test_q8ta_unary.cpp) with reference implementation and coverage across multiple shapes and quantized layouts

This diff was authored with Claude.

Differential Revision: D93511629

[pytorch-bot]

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❌ 4 New Failures, 1 Unrelated Failure

As of commit 9189369 with merge base 7b843e4 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 1
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ee13b9e20f7b748b966f3cccea9503c4d8191514972e604e15dd24087d7e9a9d /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 6cd2d8f3bc5ae8246ad9ee8f52d0cfc522ec20f4ea4227f8864b4edf1e9eed4e /exec failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test CUDA Builds / test-models-cuda (mv3) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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