fix(dspy): fixed bug in deeplake_rm retriever part

[efenocchi]

This commit fixes a bug in the DeeplakeRM class, which is a retriever model that uses Deep Lake to return the top context for a given query. The following changes have been made:

• Updated the import of OpenAI errors.
• Updated the embedding function.
• Modified the vector store instance initialization: it was instantiated every time the user made a request, now it is instantiated only at the beginning.

[arnavsinghvi11]

Thanks @efenocchi ! should be ready to merge after you run ruff check . --fix-only and push again.

[efenocchi]

thanks @arnavsinghvi11, done