batch loglikelihood_rolling across requests

[baberabb]

Currently, loglikelihood_rolling processes each request's windows separately, leading to suboptimal batching and more calls to _loglikelihood_tokens than necessary. This PR optimizes the function by batching windows across different requests.

[baberabb]

Results with meta-llama/Llama-3.2-3B, max_length=2048

Before:

hf (pretrained=meta-llama/Llama-3.2-3B,max_length=2048), gen_kwargs: (None), limit: None, num_fewshot: None, batch_size: 16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.6243	±	N/A
		none	0	byte_perplexity	↓	1.5414	±	N/A
		none	0	word_perplexity	↓	10.1131	±	N/A

vllm (pretrained=meta-llama/Llama-3.2-3B,max_length=2048,gpu_memory_utilization=0.5), gen_kwargs: (None), limit: None, num_fewshot: None, batch_size: 16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.6243	±	N/A
		none	0	byte_perplexity	↓	1.5414	±	N/A
		none	0	word_perplexity	↓	10.1138	±	N/A

After:

hf (pretrained=meta-llama/Llama-3.2-3B,max_length=2048), gen_kwargs: (None), limit: None, num_fewshot: None, batch_size: 32

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.6243	±	N/A
		none	0	byte_perplexity	↓	1.5415	±	N/A
		none	0	word_perplexity	↓	10.1153	±	N/A

vllm (pretrained=meta-llama/Llama-3.2-3B,max_length=2048,gpu_memory_utilization=0.5), gen_kwargs: (None), limit: None, num_fewshot: None, batch_size: 16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.6243	±	N/A
		none	0	byte_perplexity	↓	1.5414	±	N/A
		none	0	word_perplexity	↓	10.1138	±	N/A

[MaxiBoether]

Also wanted to give my thanks here for adressing this so quickly! Looking forward to this being merged into main :)