Eliminate 2 gpu ops during sampling when logit_bias is zero

[Qubitium]

Reason for PR: Eliminate 2 gpu ops during sampling when logit_bias is not used (all zeros)

1. skip allocation of logit_bias
2. skip applying logits.add_(self.logit_bias)

On our internal test of Yi-6B quantized using marlin on 4090 we see tangible improvement: up to 63% throughput improvement for small sized tokens output work loads when batch > 1.

Concurrency is 10 requests so it will cause sglang to use batch size between 1-10:

[hnyls2002]

@Qubitium Sorry to cause trouble. I have directly pushed the commit on your branch.
closes #344