Add implementation of Gaussians and their products

[loreloc]

Currently there are unit tests of products of two and three PCs over Gaussians verifying the correctness of the output and partition function computation.

This has been implemented by having symbolic and compiled Gaussians that can possibly be unnormalized via the specification of a log_partition parameter.

The notebook moons has been updated as to use the new pipeline apis.

Closes #210 #233 #185

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.83333% with 34 lines in your changes missing coverage. Please review.

Project coverage is 77.08%. Comparing base (01a0e24) to head (c38021e).

Files	Patch %	Lines
cirkit/symbolic/layers.py	66.66%	6 Missing and 4 partials :warning:
cirkit/symbolic/operators.py	68.75%	6 Missing and 4 partials :warning:
cirkit/backend/torch/layers/input/ef.py	76.92%	5 Missing and 4 partials :warning:
cirkit/symbolic/registry.py	33.33%	4 Missing :warning:
cirkit/backend/torch/parameters/parameter.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #234 +/- ## ========================================== + Coverage 75.58% 77.08% +1.49% ========================================== Files 44 44 Lines 2937 3137 +200 Branches 386 413 +27 ========================================== + Hits 2220 2418 +198 + Misses 635 627 -8 - Partials 82 92 +10 ```

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