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[bot]]

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 :warning:
cirkit/symbolic/layers.py	74.15%	15 Missing and 8 partials :warning:
cirkit/backend/torch/layers/input.py	70.66%	18 Missing and 4 partials :warning:
cirkit/symbolic/circuit.py	71.79%	22 Missing :warning:
cirkit/symbolic/functional.py	79.36%	7 Missing and 6 partials :warning:
cirkit/backend/torch/graph/modules.py	67.64%	8 Missing and 3 partials :warning:
cirkit/backend/torch/parameters/nodes.py	86.56%	9 Missing :warning:
cirkit/symbolic/operators.py	57.14%	9 Missing :warning:
cirkit/pipeline.py	11.11%	8 Missing :warning:
cirkit/symbolic/dtypes.py	55.55%	2 Missing and 2 partials :warning:
... and 8 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #271 +/- ## ========================================== + 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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