Implementation of litellm.batch_completion, litellm.batch_completion_models, litellm.batch_completion_models_all_responses

Doc: https://docs.litellm.ai/docs/completion/batching

LiteLLM Python SDK allows you to:

1. litellm.batch_completion Batch litellm.completion function for a given model.
2. litellm.batch_completion_models Send a request to multiple language models concurrently and return the response as soon as one of the models responds.
3. litellm.batch_completion_models_all_responses Send a request to multiple language models concurrently and return a list of responses from all models that respond.