VertexGenerationModelHelper Default model `gemini-pro` no longer works

[jsondai]

In the demo, the default model used by VertexGenerationModelHelper is gemini-pro. This model_name is not supported any more and it leaves the demo notebook stuck at the running step.

Similarly for VertexEmbeddingModelHelper, it uses 'textembedding-gecko@003' by default, and it shows 400 errors "project is not allowed to use Publisher Model"