add parallel sampling using vllm

[daspartho]

close #370

adds support for parallel sampling using vllm library when num_return_sequences in generation kwargs is > 1 and the model is supported by vllm (currently all hf models in llm-vm)

TODO: handle dependencies

[daspartho]

made suggested changes. vllm_support is set to true by default and needs to be set false explicitly for unsupported models.