use constant of default revision instead of empty root

[cr-klarna]

Fixes #3681

As mentioned in the issue, since Transformers' latest compatible non-prerelease manifest specifies < 1.0 for huggingface_hub, I believe this should be applied to the minor versions below that as well.

[cr-klarna]

This fixes a smaller problem, but there is still the fact that there is not really a proper static way that pathing is done when revision is not supplied, and the caching behavior is a bit wonky. Will need to work more on this to fully solve all issues, though this should be enough I think to give staff some inspiration to look into this a bit more so folks with private mirrors will be able to use the models more seamlessly

[bot-ci-comment]

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[hanouticelina]

thanks @cr-klarna for the PR! indeed, explicitly including /revision/main when no revision is specified is consistent with how we handle revisions elsewhere in the codebase and it ensures compatibility with mirrors that don't implement the implicit redirect
I pushed a commit to update dataset_info and space_info as well

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.50%. Comparing base (1daa48b) to head (f54d92f).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3682      +/-   ##
==========================================
+ Coverage   75.00%   77.50%   +2.49%     
==========================================
  Files         145      149       +4     
  Lines       13978    14654     +676     
==========================================
+ Hits        10484    11357     +873     
+ Misses       3494     3297     -197     

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