fix: add missing logit index increment in sampling metadata prep

[AlpinDale]

This little oversight was causing output quality to tank.

aphrodite run NousResearch/Meta-Llama-3.1-8B-Instruct --max-model-len 2048 -tp 2 --enable-chunked-prefill --enable-prefix-caching --max-seq-len-to-capture 2048

lm_eval --model local-completions --tasks gsm8k --model_args model="NousResearch/Meta-Llama-3.1-8B-Instruct",base_url=http://localhost:2242/v1/completions,num_concurrent=256,max_retries=3,tokenized_requests=False

Main:

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.0099	±	0.0027
		strict-match	5	exact_match	↑	0.0000	±	0.0000

PR:

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.7748	±	0.0115
		strict-match	5	exact_match	↑	0.7483	±	0.0120