Fix vLLM integration

[saattrupdan]

When integrating Outlines with vLLM I faced the following issues, which are fixed in this PR:

1. When calling vllm.LLM.generate then within the internals of vLLM a copy.deepcopy of the vLLM SamplingParams is made, which includes the logits processor from Outlines (RegexLogitsProcessor, say). This requires everything to be pickleable, and the RegexLogitsProcessor.fsm.vocabulary is a dict_values object, which doesn't satisfy that. The fix is easy: just convert it to a list. This doesn't affect how this vocabulary variable is being used in the code.
2. The RegexLogitsProcessor takes an llm argument, which the docstring states should be a vllm.LLM object, but then attempts to extract the underlying tokenizer via llm.tokenizer.tokenizer. The tokenizer of vllm.LLM currently lies in the vllm.LLM.llm_engine.tokenizer.tokenizer attribute, but this is a big mess and isn't backwards compatible with previous vLLM versions. Instead, they have a convenience method, vllm.LLM.get_tokenizer, which fetches the tokenizer. To remain backwards compatibility, in case people have supplied vllm.LLM.llm_engine directly into RegexLogitsProcessor, it falls back to a tokenizer or tokenizer.tokenizer attribute.

I also updated the vLLM example script, as that was outdated as well (used the previous _patched_apply_logits_processors).

Closes #704

[rlouf]

Thank you for taking the time to submit a PR. This looks great!