[Performance][Spec Decode] Optimize ngram lookup performance

[LiuXiaoxuanPKU]

After benchmarking the performance of ngram in vllm, it seems that the proposal time is longer than expected. The main reason is that there are (1) CPU <-> GPU communication when building the ngram lookup table. (2) Building the ngram contains many small kernels (duration < 5 microseconds) as show below:

Zoom in the propose time:

The PR tries to
(1) perform lookup operation on CPU.
(2) trigger CPU <-> GPU communication only when there is a match in lookup.

Some performance numbers on a single H100:
input_len: 550, output_len: 150
I changed the prompt to try different system efficiency (which might include the number of CPU <-> GPU sync).

System efficiency	propose time before this PR	propose time after this PR	end2end latency before this PR	end2end latency after this PR
0.31	4.4ms	2.2ms	6.4s	5.6s
0.63	3.3ms	1.5ms	3.8s	3.2s
0.80	2.6 ms	1.5ms	3.0s	2.6s

input_len: 2048, output_len: 150

System efficiency	propose time before this PR	propose time after this PR	end2end latency before this PR	end2end latency after this PR
0.30	6.00ms	4.54ms	9.83s	9.25s
0.63	2.90ms	2.70ms	5.84s	5.45s

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[comaniac]

LGTM!

[comaniac]

btw does this approach still have speedup if the prompt length is much longer? I'm just thinking about the trade off between CPU-GPU sync overhead and (maybe) slower CPU computation.

[LiuXiaoxuanPKU]

btw does this approach still have speedup if the prompt length is much longer? I'm just thinking about the trade off between CPU-GPU sync overhead and (maybe) slower CPU computation.

Yeah will do more benchmarks here

[mgoin]

I think this is worth considering just for the aspect of simplicity. It could even make sense to write a CPU kernel in C++ instead of trying to do it on GPU

[LiuXiaoxuanPKU]

Will change the PR so that we can change the device based on the sequence length.