Allow overriding a baseUri on GenerativeModel

[natebosch]

Allow the Vertex AI SDK to be implemented on top of GenerativeModel
without exposing any change to end users. Add a side method to create a
GenerativeModel with a specific base URI. The model code and task will
be appended.

• Add a createModelWithBaseUri top level method which is not exported
  outside of the src/ library.
• Rename _baseUrl to _googleAiBaseUri to disambiguate against the
  _baseUri field. Use "uri" over "url" more consistently.

[devoncarew]

👍 - this looks like a good solution.

I'm less familiar with the differences between the gemini and vertex protocols. If they are similar-but-different (slightly more possible return codes for block reasons? slightly different safety rating values?) we may have more work to do on the parsing side to make this SDK reusable. But allowing the vertex SDK to re-use much of the gemini one seems like a good goal.

[cynthiajoan]

👍 - this looks like a good solution.

I'm less familiar with the differences between the gemini and vertex protocols. If they are similar-but-different (slightly more possible return codes for block reasons? slightly different safety rating values?) we may have more work to do on the parsing side to make this SDK reusable. But allowing the vertex SDK to re-use much of the gemini one seems like a good goal.

Yeah that could be possible to be more diverge going forward, but for the current supported APIs the reuse is a more efficient approach. With the current setup we should be able to add logic if there starts to have more different behavior like response parsing, and when we reach the state if it's too complicated, we can replace the underneath implementation details without affecting Vertex SDK public apis.

[natebosch]

No changelog entry since this isn't user facing. I filed #111 to resolve this coupling point. I'll coordinate changes with @cynthiajoan around this method - I do think it may be necessary to change from a single Uri to a Uri Function(String apiVersion) since we will require overriding the API version for function calling.