Introduce polynomial layer and differentiation

[lkct]

As per title.

• Poly layer supports mult, conj, diff.
  • with tests: symb for all, tensor for mult and diff
  • test with both sum-prod and complex-lse
• Diff supports poly.

Minor fixes: typos; comments in pytest fixture; signature of allclose in tests

Closes #232

[codecov]

Codecov Report

Attention: Patch coverage is 76.96629% with 41 lines in your changes missing coverage. Please review.

Project coverage is 71.01%. Comparing base (3a6a98e) to head (77b4a80).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
cirkit/backend/torch/layers/input.py	73.68%	6 Missing and 4 partials ⚠️
cirkit/symbolic/layers.py	66.66%	4 Missing and 4 partials ⚠️
cirkit/backend/torch/parameters/nodes.py	80.00%	4 Missing and 3 partials ⚠️
cirkit/symbolic/operators.py	70.00%	3 Missing and 3 partials ⚠️
cirkit/pipeline.py	33.33%	4 Missing ⚠️
cirkit/symbolic/functional.py	88.23%	2 Missing and 2 partials ⚠️
cirkit/symbolic/parameters.py	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #272      +/-   ##
==========================================
+ Coverage   70.68%   71.01%   +0.32%     
==========================================
  Files          45       45              
  Lines        4575     4744     +169     
  Branches      668      703      +35     
==========================================
+ Hits         3234     3369     +135     
- Misses       1161     1176      +15     
- Partials      180      199      +19     

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