Allow sparse matrix as circuit input

[royess]

Issue Description

In some cases, we would like a sparse matrix as circuit input.

Right now, this seems impossible because of type conversion. For example, DMCircuit does this:

dminputs = backend.convert_to_tensor(dminputs)

which throws an error like "TypeError: Failed to convert elements of SparseTensor(...) to Tensor" in TensorFlow.

Proposed Solution

I'm not sure whether we can just modify type conversions.