Update model_checkpoint's to_yaml to use fsspec open

[ananthsub]

What does this PR do?

Same spirit as #3782. Use fsspec consistently to support more storage backends. cc @awaelchli

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[Borda]

lgtm

[f4hy]

Thanks! I also need this.

[codecov]

Codecov Report

Merging #3801 into master will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #3801    +/-   ##
=======================================
+ Coverage      85%     89%    +4%     
=======================================
  Files         110     110            
  Lines        8536    8649   +113     
=======================================
+ Hits         7293    7735   +442     
+ Misses       1243     914   -329