Add examples and documentation for using custom model providers

[rm-openai]

Did a few things:

1. Lazy loaded the client in OpenAIProvider so that we don't raises errors in cases where you dont have an openai key, but an OpenAIProvider() is created.
2. Added docs and examples for custom model providers.

Test plan:

export EXAMPLE_BASE_URL="..."
export EXAMPLE_API_KEY="..."
export EXAMPLE_MODEL_NAME"..."

Then run the examples, e.g.:

python examples/model_providers/custom_example_provider.py

Loops within themselves,
Function calls its own being,
Depth without ending.