add time to first token in OnnxDiscrepancyCheck

[xadupre]

Describe your changes

• add metrics time to first token, time to first 5 tokens in OnnxDiscrepancyCheck

[Copilot]

Pull request overview

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Adds time-to-first-token and time-to-first-N-tokens latency metrics to OnnxDiscrepancyCheck’s generation comparison output, and updates tests accordingly.

Changes:

• Introduces a new time_to_first_n_tokens config option and surfaces latency metrics for both Transformers and ORT GenAI generation.
• Changes compare_generation to return a results dictionary (instead of an int) and updates result aggregation/logging.
• Updates ONNX discrepancy check tests to validate the new return shape and metric keys.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
test/passes/onnx/test_discrepancy_check.py	Adjusts assertions for the new dict return type and validates presence of new latency metric fields.
olive/passes/onnx/discrepancy_check.py	Adds latency measurement/reporting for TTFT/TTFN and changes generation comparison to return structured results.