llms/googleai: batch embedding calls

[johanbrandhorst]

The new Batch API significantly speeds up embedding

PR Checklist

• [ ] Read the Contributing documentation.
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• [ ] Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
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[tmc]

So good to see you contributing! can't wait to get this in!.

[johanbrandhorst]

Long time no see Travis :). Well done on this project. I'm wondering how you would prefer I test this, the only way I can see is to spin up a custom HTTP server to fake the requests but it doesn't provide much in the way of confidence - I have tested this personally against the Gemini API and it works great.

[eliben]

Also happy to see you contribute here, @johanbrandhorst :-)

Re tests, the googleai provider is relatively well-tested with live tests vs. the Gemini API (https://github.com/tmc/langchaingo/blob/main/llms/googleai/shared_test/shared_test.go)

For this test specifically, testing batching vs. no batching is a bit tricky and we don't really do mock testing with fake HTTP backends for now. That said, if the shared live tests pass and you observe the performance improvement, I believe this is good enough.