[devtools] Add preprocessing support to NumericalComparatorBase

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [devtools] Add reference_graph parameter to calculate_numeric_gap #17459
• [devtools] Auto-record after-transform graph in ETRecord #17434
• -> [devtools] Add preprocessing support to NumericalComparatorBase #17433

This change extends NumericalComparatorBase to support custom preprocessing before numeric gap calculation, enabling backends like Qualcomm to apply necessary tensor transformations (e.g., dequantization, layout conversion) before comparison.

Key changes:

• Extended NumericalComparatorBase with:
  • __init__(inspector) to store optional Inspector reference for accessing graph metadata
  • preprocessing(mapping) method that can be overridden for custom tensor transformations (default: identity)
  • element_compare(a, b) abstract method for element-level tensor comparison
  • compare(mapping, ...) method that orchestrates the full pipeline: preprocessing → element-wise compare → aggregate to DataFrame

This is part of the operator-level numeric discrepancy detector project for ExecuTorch Qualcomm backend (#16381).

Design doc: https://docs.google.com/document/d/1GaCHiy9InytOsUrl2BKEgOiP1iKTfpCVdWg6QDh0N2E/edit?tab=t.0#heading=h.fcrpnrtb6cud

Differential Revision: D93169813

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17433

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❌ 3 New Failures

As of commit 73d8ae3 with merge base 6e31609 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for devtools/inspector/tests/inspector_utils_test.py:
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 6ff735d5b9f55ebde9dc958251995764c065567876dfda01f1ac91e27af0af36 /exec failed with exit code 139
• pull / test-samsung-models-linux / linux-job (gh)
  test_inception_v3_fp16

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