Forced function calling not working with Gemini Pro 1.5 and OpenAI library

[levisz]

Hey, 

I'm using the Openai library to do some function calling requests with the Gemini models. Based on the documentation (https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/call-gemini-using-openai-library#cl...) the Pro 1.5 model should support forced function calling, but whenever I try adding 'required' or 'any' as tool_choice I'm getting this error: 

Error code: 400 - [{'error': {'code': 400, 'message': "Expected a string 'tool_choice' to be one of 'none' or 'auto'; found 'required'.", 'status': 'INVALID_ARGUMENT'}}]

I've tried both  'google/gemini-1.5-pro-preview-0409' and 'google/gemini-1.5-pro-001' as models.  It does work with 'auto' so the rest of the call should be ok.