fix: Decouple exposed_generation time from weight_sync time

[youngeunkwon0405]

What does this PR do ?

Example output after this fix

📊 Training Results:
  • Loss: 0.1032
  • Avg Reward: 0.3486
  • Buffer Size: 64
  • Avg Trajectory Age: 1.00 steps

⏱️  Timing:
  • Total step time: 66.55s
  • exposed_generation: 32.52s (48.9%)
  • policy_training: 25.05s (37.6%)
  • policy_and_reference_logprobs: 4.78s (7.2%)
  • buffer_sampling: 1.38s (2.1%)
  • logprob_inference_prep: 1.34s (2.0%)
  • training_prep: 1.06s (1.6%)
  • data_processing: 0.23s (0.4%)
  • weight_sync: 0.13s (0.2%)

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