fix: support missing torch with spark and multiprocessing backends

[percevalw]

Description

Fixes #243

• one of the errors came from torch being imported without handling import errors (torch is an optional dependency)
• the second main error came from the from .registry import registry in edsnlp/core.py, preventing python from programmatically importing the classes from edsnlp/core/registry.py. Because of this, dill tryed to serialize the entire registry module, and some other modules from edsnlp, causing various serialization issues.

Checklist

• If this PR is a bug fix, the bug is documented in the test suite.
• Changes were documented in the changelog (pending section).
• If necessary, changes were made to the documentation (eg new pipeline).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eb08237) 96.70% compared to head (2da3cc3) 96.70%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #244   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files         255      255           
  Lines        8369     8372    +3     
=======================================
+ Hits         8093     8096    +3     
  Misses        276      276           

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