Make download scripts more accessible

[philipp-zettl]

Sources for models/architectures/pipelines are scattered all over the project.
We must unify and simplify the process for users/admins.

In order to do that, port all downloads to the "new" huggingface_hub.snapshot_download method.

Here's a quick example for the ONNX version of Phi3-4k:

from huggingface_hub import snapshot_download

snapshot_download(repo, allow_patterns=['cpu_and_mobile/cpu-int4-rtn-block-32-acc-level-4/*'], local_dir='models/cpu_and_mobile/phi-3-4k')