Missing docstrings

[IrwinChay]

Currently many class and function docstrings are missing, notice that they would also affects the documentation in https://cirkit-docs.readthedocs.io/

These includes

cirkit/pipeline.py
cirkit/utils/algorithms.py
cirkit/backend/compiler.py
cirkit/backend/registry.py
cirkit/backend/torch/compiler.py
cirkit/backend/torch/initializers.py
cirkit/backend/torch/optimization/registry.py
cirkit/backend/torch/graph/optimize.py
cirkit/backend/torch/graph/folding.py
cirkit/backend/torch/parameters/pic.py
cirkit/backend/torch/parameters/nodes.py
cirkit/backend/torch/parameters/optimized.py
cirkit/backend/torch/rules/initializers.py
cirkit/backend/torch/rules/parameters.py
cirkit/backend/torch/rules/layers.py
cirkit/templates/region_graph/graph.py
cirkit/symbolic/registry.py
cirkit/symbolic/operators.py