Fixed bugs and add lib support for implementing (sum of) squared circuits + new notebooks

[loreloc]

• Add notebooks showing how to implement squared circuits and sum of squares circuits (without torch graph optimizations for now).

• Add complex valued <log-sum-exp, sum> semiring implementation + refactored other semirings as to enable promotion of tensor dtypes, based on the dtype needed by the semiring itself.

• The semiring abstract class now supports dynamic dispatching of new mappings between semirings (i.e., morphisms). This is necessary to enable the user to write new semiring implementations.

• Add 'merge' symbolic operator, allowing us to combine multiple symbolic circuits into a single one having multiple outputs.

• Fixed a number of issues related to folding:

  1. fixed a corner case preventing the folding algorithm to return more optimized fold index information: i.e., it returned fold indices that were equivalent to tensor unsqueezes.
  2. fixed a corner case where folding could not been performed when parameter computational graphs had multiple outputs that should have been stacked together. This resulted in exceptions being raised for some symbolic circuits.
  3. reduced cpu communication overhead by moving fold index information to the device the circuit is actually on.

• Re-added clamping parameter node.

• Fixed wrong dependencies in pyproject.toml and upgraded to Python 3.9 for much better and faster utilities regarding functools and similar std libs.

Closes #235 #207 #193 #185 .

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.58940% with 116 lines in your changes missing coverage. Please review.

Project coverage is 71.57%. Comparing base (3270d77) to head (4cc50b6).

Files	Patch %	Lines
cirkit/backend/torch/semiring.py	64.42%	49 Missing and 4 partials :warning:
cirkit/symbolic/functional.py	37.50%	19 Missing and 1 partial :warning:
cirkit/pipeline.py	0.00%	12 Missing :warning:
cirkit/backend/torch/parameters/ops.py	35.29%	11 Missing :warning:
cirkit/symbolic/parameters.py	33.33%	10 Missing :warning:
cirkit/backend/torch/graph/modules.py	76.92%	1 Missing and 2 partials :warning:
cirkit/backend/torch/layers/sum_product.py	0.00%	2 Missing :warning:
cirkit/backend/torch/rules/parameters.py	33.33%	2 Missing :warning:
cirkit/symbolic/circuit.py	33.33%	1 Missing and 1 partial :warning:
cirkit/backend/base.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #237 +/- ## ========================================== - Coverage 73.09% 71.57% -1.52% ========================================== Files 46 46 Lines 3319 3504 +185 Branches 441 471 +30 ========================================== + Hits 2426 2508 +82 - Misses 801 900 +99 - Partials 92 96 +4 ```

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