Refactor embedding model name in call

[Surya2k1]

Description of the change

Embedding model requires to mention as "models/<model_name>" but not other generative models which is simply "<model_name>"

Motivation

It just for consistency of model usage. May fix #640.

Type of change

Feature request

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.

[Surya2k1]

CC: @MarkDaoust