Evidence operator

[loreloc]

Implemented evidence symbolic operator and added support for it in the Torch compiler.

List of major changes in this PR:

• Previously, we had that integrating a circuit yields a circuit with the same scope, but this should not be the case. For instance, if I integrate Y in a circuit computing c(X,Y), I should receive a circuit over X only. To fix this problem, I have made the LogPartitionLayer (computing the log partition function of an input layer) to have an empty scope instead.
• The symbolic evidence operator is implemented by encapsulating an input layer in a symbolic EvidenceLayer whose constant parameters consists of an assignment to the variables the input layer is defined on. Similarly, to the LogPartitionLayer, this EvidenceLayer has empty scope.
• Furthermore, previously we had that the scope was a property of each symbolic layer. However, since the connections are stored in the circuit, the circuit itself now stores a mapping between layers and their scope, which is pre-computed up to initialization. Given a symbolic circuit, the scope of a layer can be retrieved with the method layer_scope(.).
• Added the implementation of the product of circuits and sub-circuits over disjoint (i.e., non-overlapping) scopes. However, it only works when layers of the same size and over non-overlapping scopes are being multiplied.

Closes #268 .

[codecov]

Codecov Report

Attention: Patch coverage is 78.00270% with 163 lines in your changes missing coverage. Please review.

Project coverage is 70.16%. Comparing base (e5f1b5c) to head (f9bdb0f).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
cirkit/symbolic/parameters.py	82.55%	25 Missing and 1 partial ⚠️
cirkit/symbolic/layers.py	74.15%	15 Missing and 8 partials ⚠️
cirkit/backend/torch/layers/input.py	70.66%	18 Missing and 4 partials ⚠️
cirkit/symbolic/circuit.py	71.79%	22 Missing ⚠️
cirkit/symbolic/functional.py	79.36%	7 Missing and 6 partials ⚠️
cirkit/backend/torch/graph/modules.py	67.64%	8 Missing and 3 partials ⚠️
cirkit/backend/torch/parameters/nodes.py	86.56%	9 Missing ⚠️
cirkit/symbolic/operators.py	57.14%	9 Missing ⚠️
cirkit/pipeline.py	11.11%	8 Missing ⚠️
cirkit/symbolic/dtypes.py	55.55%	2 Missing and 2 partials ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
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+ Coverage   68.95%   70.16%   +1.20%     
==========================================
  Files          45       45              
  Lines        4986     5238     +252     
  Branches      547      578      +31     
==========================================
+ Hits         3438     3675     +237     
+ Misses       1348     1346       -2     
- Partials      200      217      +17     

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