Add a loading model function for Ollama classes

[Frank995]

Checked

• I searched existing ideas and did not find a similar one
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• I've clearly described the feature request and motivation for it

Feature request

Ollama classes (like Chat and Embedding), should have an option to load the model, potentially accepting the model id as parameter.

Motivation

In code I'm using LangChain for every model related query, but I need to explicitly call requests when loading the model in advanced, which is not ideal, especially since the backbone for doing it is already in langchain

Proposal (If applicable)

I drafted a PR for that: #24387