feat: improve overall prediction accuracy through separating prefill and decode.

[weizhehuang0827]

• Separate time prediction for prefill and decode sequences.

• The constant term in linear regression does not accumulate as the number of sequences in a batch increases.

• Prediction accuracy test:

  • In the prefill-only scenario, MAPE error: ~12% -> ~8%
  • In the decode-only scenario, MAPE error: very large -> ~5%
  • In the mixed prefill and decode scenario, MAPE error: very large -> ~7%