[feat] Support custom filesystems in LightningModule.to_torchscript

[ananthsub]

What does this PR do?

This code wasn't using fsspec meaning remote filepaths (azure, s3, gcs) weren't supported out of the box. This PR fixes that by using fsspec, similar to how we use this in other parts of the codebase..

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[codecov]

Codecov Report

Merging #7617 (b41fcf9) into master (608de6a) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7617    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         197     198     +1     
  Lines       12871   12937    +66     
=======================================
- Hits        11871   11375   -496     
- Misses       1000    1562   +562     

[ananthsub]

@carmocca @tchaton @Borda mind taking a look when you get a chance?

[carmocca]

Are these set as variables (instead of hardcoded) for a reason? Do you expect that other tests would need to modify them?

[ananthsub]

i think a dummy custom filesystem could be helpful to ensure fsspec works as expected, but this is not the right spot to land this. for now, i will move it into the test. i initially avoided putting this into the test to avoid registering the filesysem 3 times

[tchaton]

LGTM !