(transform): csl_stencil canonicalization pass

[n-io]

Adds a canonicalisation pass for csl_stencil.apply. The op takes an empty tensor asiter_arg, which it does not manage itself. The conversion pass in #2803 initialises an iter_arg for each instance of apply. This canonicalisation pass identifies where this can be re-used, effectively removing redundant allocations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.83%. Comparing base (01640fc) to head (59a38f8).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2814      +/-   ##
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+ Coverage   89.82%   89.83%   +0.01%     
==========================================
  Files         396      398       +2     
  Lines       49055    49213     +158     
  Branches     7510     7552      +42     
==========================================
+ Hits        44063    44210     +147     
- Misses       3803     3808       +5     
- Partials     1189     1195       +6     

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