feat: make onnx export a function

[stephantul]

This PR makes the ONNX export we offered a part of the model2vec package. I added extra things to the onnx extra to support new torch versions and made the script work on an in-memory version of a model, not one stored on disk.

from model2vec import StaticModel
from model2vec.onnx import export_model_to_onnx

s = StaticModel.from_pretrained("minishlab/potion-base-8m")
export_model_to_onnx(s, "my_model")

This writes an onnx model to disk, which can then be uploaded to Hugging Face manually and used wherever. I chose to write some additional config options which were needed by fastembed. Most of these are not necessary, which I reported here.