[atom-vllm] use aiter greedy sampler

[zejunchen-zejun]

use aiter greedy sampler to replace the vLLM original greedy sampler
accuracy status:

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	3	exact_match	↑	0.9242	±	0.0073
		strict-match	3	exact_match	↑	0.9234	±	0.0073

with aiter sampler(1k1k con8):

without aiter sampler(1k1k con 8):

with aiter sampler(1k1k con32):

without aiter sampler(1k1k con 32):