Added support for variable sized output vectors via Nomic Embed v1.5

[AndriyMulyar]

Added support for Nomic Embed v1.5: https://blog.nomic.ai/posts/nomic-embed-matryoshka

This let's you specify a vectorizer that will generate much smaller but still performant embeddings for local use.

[okhat]

Wonderful, thanks a lot. Merging after checks.

[okhat]

Ok we have a small problem:

import nomic

I guess normally we deal with this by importing optional dependencies either:

1. Inside the class
2. Outside the class, but in try/except form and on except just set it to None or something

[AndriyMulyar]

Will update.

[arnavsinghvi11]

Hi @AndriyMulyar , just following up on this update. looks good to merge after editing the import. Feel free to use examples from other retrievers on how to optionally import the local dependencies