fix: ollama embedding_model dims Option panic.

[KanonWY]

use ollama embedding_model， Missing model dimensions.

    let ollama_client = ollama::Client::from_env();
   // this will panic.
    let model = ollama_client.embedding_model("nomic-embed-text");

add function model_dimensions_from_identifier to give a default dimension.
maybe fix 1189

[joshua-mo-143]

Looks OK. See my comment

[joshua-mo-143]

I think this part is largely unmaintainable, so we should avoid adding this in. It is usually quite trivial to find the dimension sizes

[KanonWY]

yes, I think this part is largely unmaintainable too. A better approach is to use embedding_model_with_ndims. I added it this way because I noticed that other modules did the same. openai/embedding.rs:row59