🔧 chore(loading.py): add instantiate_llm function to handle LLM instantiation and set ChatConfig.streaming based on openai_api_base parameter

This commit adds a new function `instantiate_llm` to handle LLM (Language Model) instantiation. It also sets the `ChatConfig.streaming` attribute based on the `openai_api_base` parameter. This is a workaround to ensure that JinaChat works until streaming is implemented.