[ET-VK][q8ta] Add q8ta_linear_gemv op for batch-1 int8 linear

[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

Add a cooperative GEMV variant of q8ta_linear optimized for batch size 1. The existing q8ta_linear uses a tiled algorithm with 4H4W packed int8 layout, which is inefficient for single-row inputs because it wastes 3/4 of each ivec4 block. The new q8ta_linear_gemv uses 4W packed int8 layout (scalar int[] buffers) and a cooperative algorithm where 64 threads split the K reduction dimension with shared memory tree reduction.

The shader loads one packed int32 (4 int8 values) per thread per K iteration and accumulates dot products against the weight tile using dotPacked4x8AccSatEXT. After reduction, thread 0 applies scales, zero points, bias, and quantizes the output.

The pattern matcher in quantized_linear.py selects q8ta_linear_gemv when the input batch dimension is 1, falling back to q8ta_linear for larger batches.

Also adds PACKED_INT8_4W (value 5) to the serialization schema to support the 4W memory layout in the export pipeline.

Authored with Claude.

Differential Revision: D93768643

[pytorch-bot]

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❌ 4 New Failures, 1 Unrelated Failure

As of commit 5c5dd83 with merge base 9a58ce8 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2e173131eb706d5a1742d243bef482ab0ad194dad6c42050016d71eeaf66adab /exec failed with exit code 2
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a50b1ff2e44009863d19dd0362877e0f6d66ed257fd0caab8df80060bf8e8784 /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  test_edsr_fp16
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6bb83453aded1a5abe04680f6e2e64c76a9b15a01898ec885cfbf5c3e4a46a3a /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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This pull request has been merged in 187dc1d.