Add vllm LLM subclass

[fadil4u]

Added vllm subclass.

Usage

llm = VLLM("mistralai/Mistral-7B-Instruct-v0.1",api_base=api_base)

mapped = dt.map(
    prompt=mapping_prompt,
    output_fields=["passenger_trend_comment"],
    input_fields=["year","month","passengers"],
)(llm, df)