[Frontend] Cleaning up callback code

[erick-xanadu]

Context: Necessary refactoring for fwd and bwd passes.

Description of the Change: Makes a FlatCallable and MemrefCallable.

[sc-63354]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.30769% with 4 lines in your changes missing coverage. Please review.

Project coverage is 98.06%. Comparing base (0724db4) to head (0268382). Report is 177 commits behind head on main.

Files	Patch %	Lines
frontend/catalyst/api_extensions/callbacks.py	92.30%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #742      +/-   ##
==========================================
- Coverage   98.10%   98.06%   -0.04%     
==========================================
  Files          70       70              
  Lines        9570     9585      +15     
  Branches      753      753              
==========================================
+ Hits         9389     9400      +11     
- Misses        147      151       +4     
  Partials       34       34              

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.