Fix fetching models from example-models repo

[vloncar]

Description

fetch_example_model and fetch_example_list use outdated links so they no longer work. This PR fixes that. I made the organization and branch as variables so we can override them for debugging if needed. Also, this function expects .json +.h5 of weights, but we usually don't ask users to do this, so I added support for fetching models that are a single .h5 file.

Type of change

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Note: Please delete options that are not relevant.

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality) -- sort of "new feature"

Tests

Do we want tests of this in pytest?

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.