Update first token time calculation in cuda benchmark

[Gasoonjia]

previously we onlu calculate the prefill time as first token time. However we should calculate the time from the model start, to the time we get the first token.

this pr updates the calculation to solve the issue

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job, 1 Unrelated Failure

As of commit 597c2ce with merge base abe3a71 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t 6deacc490dafbcbb685be888631ff0264c85531c5addc78dfd80b17373dc892a /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 484ce04e8e13df2ee2a8e919dcfc08a37667d8f64f0ed954333eba4572743bb8 /exec failed with exit code 1
• trunk / test-arm-backend (test_pytest_models_ethosu_fvp) / linux-job (gh)
  RuntimeError: Command docker exec -t b7e7adb0892897ce3267eae3fb90937e88a09a2985797d6a82b63306fc8d8ca3 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

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

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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