Add utility function for retrieving locally cached models 
Author: cdpierse
Pull request: yes

# What does this PR do?

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Adds implementation of a small utility function for retrieving a list of locally cached models, discussed in #8803 



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## Who can review?

@LysandreJik

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Comments:
No problem just pushed the fix now. 

@LysandreJik  Not sure what exactly caused the flax test suite on this to crash. It looks like the docker image crashed.

I think you need to run the `make style` command on your branch to fix the styling issues. The other test failures seem spurious.

Thanks!