[b/307966230] Implement force_download option for model_download over HTTP

[lucyhe]

Previously, after a model or file was downloaded with model_download, there was no way to programmatically force a new download. This PR enables forced downloads.

More Details

• This feature can only be used outside of Kaggle notebooks. Kaggle notebooks rely on a different caching mechanism where force_download is not yet possible
• force_download defaults to False
• If a file is force downloaded, the other files in the same folder are not force downloaded

New flag usage:

kagglehub.model_download(model_handle, force_download=True)
kagglehub.model_download(model_handle, path=path, force_download=True)

[lucyhe]

I refactored some tests so that the new force_download tests wouldnt feel too verbose. Please let me know if you disagree with any refactoring, and I'm happy to revert!

[rosbo]

And also, looks like shutil.rmtree is failing for some versions of Python based on whether files are there or not. See the tests run from CI.

[lucyhe]

And also, looks like shutil.rmtree is failing for some versions of Python based on whether files are there or not. See the tests run from CI.

Thanks for the quick review! Are the CI checks identical except for python version? Want to make sure I've narrowed down what could be happening