add LiteLLMVectorizer to allow more embeddings to be used

[hawktang]

With LiteLLM proxy, the embedding model that LiteLLM supported can be used in DSPY

[arnavsinghvi11]

Thanks for opening this PR @hawktang . just curious, where is the LiteLLMVectorizer being used? seems to me it is just using the openAI embeddings but just wanted to double-check here.

[hawktang]

Sorry for the late relay, I am on travel now.

LiteLLMVectorizer is used to call embedding models which LiteLLM proxy supported.

LiteLLM proxy is used to call different LLM APIs using the OpenAI API format.

With LiteLLM adaptor DSPY can direct support all the cloud and local model LiteLLM supports.

Because it is using OpenAI API format, LiteLLMVectorizer I wrote is quite similar except the base_url

Directly add base_url as parameter for openAI embeddings can achieve the result.

I will raise another PR if LiteLLMVectorizer class is redundant.