Some users would greatly benefit from a guide on creating custom model clients. We should create a user guide notebook: python/packages/autogen-core/docs/src/user-guide/extensions-user-guide/custom-model-client.ipynb to show how to implement a custom model client subclassing the autogen_core.models.ChatCompletionClient base class.

We can use huggingface transformer pipeline as the underlying model API to create an example custom model client in the user guide.

Existing user guide for using built-in model clients: python/packages/autogen-core/docs/src/user-guide/core-user-guide/components/model-clients.ipynb

Related issue: #6538