Add "google-cloud-aiplatform[prediction]" as custom requirement for Kokoro script

[dandhlee]

There is a new library in python-aiplatform that requires a special install. Filing the issue to keep track of this.

[tbpg]

What exactly does this mean? Sounds like the general docs plugin needs to install something to be able to handle a particular library? Feels like the library should have dependencies set up so that isn't necessary? I'm probably misunderstanding.

[dandhlee]

The library in question does have a specific handler for this (https://github.com/googleapis/python-aiplatform/blob/12be04ccc63926a25afef61aae9747fdb6043030/noxfile.py#L312). I need to also add it in the script when generating the docs for that particular part of the library, namely for prediction portion, in https://github.com/googleapis/sphinx-docfx-yaml/blob/main/.kokoro/requirements.in