Add post API for embedding model and fix embedding model wrappers

[ZiTao-Li]

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Add post API for embedding model and fix embedding model wrappers

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Description

• Add post API for embedding model to provide a default choice for some response format.
• Fix some bugs (which will not be trigger) in the existing embedding models.

By default, now the return of embedding model wrappers are ModelResponse with list of embeddings.

Checklist

Please check the following items before code is ready to be reviewed.

• [x] Code has passed all tests
• [x] Docstrings have been added/updated in Google Style
• [x] Documentation has been updated
• [x] Code is ready for review