Allowing users to add new operators

[lkct]

Currently, end-users cannot add a new circuit operator, as it involves modifying (instead of inheriting) some classes in the library. The original motivation is that adding a new operator requires more understanding of the internals of the library and should be done as a dev.

However, it would still be great if we allow users to do so. E.g. to implement Fisher Divergence, the current version of cirkit is still not enough -- we lack a "sum operator" for the inner product, which should be easy to impl, but still impossible with only user code.

cc @loreloc

[lkct]

Some premature idea:
Use more "registries" so that the classes are switchable by registering a subclass.

Meanwhile, we have registry override implemented, which might be useful, e.g.

cirkit/cirkit/symbolic/functional.py

Line 162 in 00c07f0

def multiply(sc1: Circuit, sc2: Circuit, registry: OperatorRegistry | None = None) -> Circuit: