Feature: Create an automated way to update reference traces

[unagiboi]

Summary

This PR adds an automated way to update the checked-in reference CSV directories directly from the test runner.

What changed

New infrastructure in tests/conftest.py

• --update-references pytest CLI flag — pass to any pytest invocation to enable update mode
• update_references fixture — injects a boolean into tests that checks both the CLI flag and the env var
• update_reference_csvs() helper — atomically replaces a reference CSV directory with freshly generated output (removes stale sheets)

Updated test modules

All regression test modules that compare generated output against checked-in reference CSV directories now accept the update_references fixture:

Test file	What it updates
test_perf_report_regression.py	*_perf_report_csvs/ directories next to .json.gz traces
test_compare_perf_reports.py	reference/ subdirectories for compare tests (basic + E2E)
test_jax_perf_report.py	*_perf_report_csvs/ directories for JAX traces
test_detect_recompute.py	resnet_act_checkpoint_recompute_perf_report_csvs/
test_collective_analysis.py	nccl_analysis_report_csvs/

Usage

# Update all references at once
pytest tests/ --update-references

# Update only PyTorch perf report references
pytest tests/test_perf_report_regression.py --update-references

# Update references used by specific test
pytest tests/test_compare_perf_reports.py::test_compare_perf_reports --update-references

When update mode is active, each test:

1. Generates the report as usual
2. Copies the generated CSV directory over the checked-in reference directory

Closes #561

[ajassani]

We should add to the code comment/docstring as well as contributing.md that
when you use this flag, explicitly note in your PR description which references were updated and why. Do not use this flag to paper over unexpected differences.