[ET-VK][testing] Add per-shader timing breakdown to benchmark output

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][qconv] Add flexible layout impl for quantized pointwise conv #17221
• [ET-VK][testing] Create dedicated test binary for pointwise convolutions #17220
• [ET-VK][qconv] Add layout-agnostic general shader for quantized conv #17219
• [ET-VK][quantization] Add layout-flexible clone for int8x4 tensors #17171
• [ET-VK][qconv] Add layout-flexible impl of quantized depthwise conv2d #17108
• [ET-VK][ez] Implement helper functions to get fastest moving dim #17107
• [ET-VK][quantization] Implement layout-flexible quantize/dequantize operators #17106
• [ET-VK] Add alignment fields to PackedDimInfo for padded size calculation #17170
• -> [ET-VK][testing] Add per-shader timing breakdown to benchmark output #17105

Previously, benchmark results only showed aggregate GPU timing for each
test case, making it difficult to identify which specific shaders were
executing and how they contributed to the overall runtime. This change
adds per-shader timing instrumentation to enable quick identification
of performance bottlenecks.

Key changes:

• Add ShaderTiming struct to track individual shader execution times
  across iterations
• Add collect_per_shader_timing_us() helper that extracts timing data
  from the querypool on a per-shader basis (vs the previous aggregate)
• Update BenchmarkResult::print_row() to display shader names and
  their individual timings. When multiple shaders participate, each
  shader's average time is shown on its own line before the summary row
• Add string utility helpers (layout_abbrev, storage_type_abbrev,
  repr_str, shape_string) for generating concise test case names
• Adjust column widths for better terminal fit

Differential Revision: D91945038

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17105

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❌ 2 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 370f2d9 with merge base 477867a ():

NEW FAILURES - The following jobs have failed:

• pull / android / build-llm-demo / linux-job (gh)
  RuntimeError: Command docker exec -t bb0685810fe46805e0b6c19e21ccc55d20623194fb488271be30a8fcde9f0336 /exec failed with exit code 1
• pull / test-vulkan-operators-linux / linux-job (gh)
  RuntimeError: Command docker exec -t cba4c9d22821fc3f4a9f7d470c7115cd8e6f888945d1c8d19c480966d6321fa4 /exec failed with exit code 134

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

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

• pull / test-samsung-quantmodels-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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