[Core] Faster logit_bias_logits_processor

[xu-song]

Changes

• Change python ops to tensor ops, which reduce time cost of logit_bias_logits_processor from 106ms to 0.01ms (when len(logit_bias)=10000).

Before(python ops)	After (tensor ops)
for token_id, bias in logit_bias.items(): logits[token_id] += bias	logits.index_add_(0,logit_bias_index, logit_bias_value)

• Add cache for logit_bias

Impl history

• v1: no cache
• v2: cache in single request
• v3: cache across different requests

Performance

len(logit_bias)	time cost (ms) before -> v1 -> v2/v3
1	4.5 -> 0.3 -> 0.01
20	4.5 -> 0.3 -> 0.01
100	5.3 -> 0.3 -> 0.01
1000	14.4 -> 0.3 -> 0.01
10000	106 -> 0.4 -> 0.01

experiment settings:

GPU: A100 
model: Llama-3.2-1B-Instruct

[github-actions]

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[imkero]

If len(logit_bias) is large, maybe we can keep the copy of logit_bias["index"] and logit_bias["value"] in the device memory ahead of time (or in the first sample step), and re-use it in the following sample steps, to avoid duplicated tensor copy?

[xu-song]

If len(logit_bias) is large, maybe we can keep the copy of logit_bias["index"] and logit_bias["value"] in the device memory ahead of time (or in the first sample step), and re-use it in the following sample steps, to avoid duplicated tensor copy?

@imkero Thanks for your suggestion, a new commit has been added, which avoid duplicated tensor copy.

After this change, the time_cost is reduced to 0.01ms

len(logit_bias)	time cost (ms)
1	4.5 -> 0.01
20	4.5 -> 0.01
100	5.3 -> 0.01
1000	14.4 -> 0.01
10000	106 -> 0.01

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @xu-song.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[xu-song]

@njhill conflicts have been resolved.