Adding support for Gemini

[shima-khoshraftar]

Hi, I added ChatVertexAI from langchain_google_vertexai to models.py file to use gemini models. However, the issue is that i am getting InvalidArgument: 400 Request contains an invalid argument error with the current code when invoking the model with a text and a schema. Just wondering if you have some insights on this as it seems that this is going to be a feature in your next release? Thanks.

[shima-khoshraftar]

It worked fine with the code for the first release of langchain-extract. I can use that until your next release. Thanks.