Add lib support for complex circuits

[loreloc]

• Add a way to specify data types of tensor parameters in the symbolic representation. The precision of these data types (e.g., float64, complex32, ...) is then chosen at compile time.

• Add conjugate circuit symbolic operator, returning the circuit computing the complex conjugate of another circuit.

Closes #240 .

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.72000% with 133 lines in your changes missing coverage. Please review.

Project coverage is 65.36%. Comparing base (60e1f7a) to head (73b2f4f).

Files	Patch %	Lines
cirkit/backend/torch/parameters/nodes.py	84.25%	59 Missing and 1 partial :warning:
cirkit/symbolic/layers.py	55.93%	13 Missing and 13 partials :warning:
cirkit/backend/torch/rules/parameters.py	47.61%	9 Missing and 2 partials :warning:
cirkit/pipeline.py	0.00%	10 Missing :warning:
cirkit/backend/torch/semiring.py	71.42%	8 Missing :warning:
cirkit/symbolic/dtypes.py	57.14%	4 Missing and 2 partials :warning:
cirkit/symbolic/circuit.py	73.33%	4 Missing :warning:
cirkit/symbolic/operators.py	77.77%	4 Missing :warning:
cirkit/symbolic/functional.py	92.00%	1 Missing and 1 partial :warning:
cirkit/symbolic/parameters.py	95.12%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #242 +/- ## ========================================== - Coverage 71.36% 65.36% -6.00% ========================================== Files 54 56 +2 Lines 4040 4487 +447 Branches 522 558 +36 ========================================== + Hits 2883 2933 +50 - Misses 1052 1439 +387 - Partials 105 115 +10 ```

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