feat: Cerebras API support.

You can now run langroid with LLMs hosted on cerebras by setting up a CEREBRAS_API_KEY in your environment,
and specifying the chat_model in the OpenAIGPTConfig as cerebras/<model_name>, e.g. cerebras/llama3.1-8b.

Cerebras docs: https://inference-docs.cerebras.ai/introduction

Guide to using Langroid with Cerebras-hosted LLMs: https://langroid.github.io/langroid/tutorials/local-llm-setup/