[Bug] Drawing certain kron operations fails due to a bug in fill_identities

[jpmoutinho]

Short description

The function fill_identities fails when doing kron(CNOT(0, 2)) because it adds one extra identity than what is needed and then the kron function fails to build the KronBlock because there is overlapping support. The function fill_identities is called by the drawing module so that the proper spacings between blocks are included in the drawing.

What is the expected result?

No response

What is the actual result?

No response

Steps/Code to reproduce

Initially noticed when trying to display an hea on a different qubit support. E.g.,

from qadence import *
from qadence.draw import display

display(hea(n_qubits = 2, support = (0, 2)))

but can be reproduced simply with

display(kron(CNOT(0, 2)))

This relates to issue #211, and is one more reason why fill_identities might need a proper refactor.

I considered fixing this by having kron(CNOT(0, 2)) just return a CNOT and not a KronBlock but that is not backwards compatible and could cause other isses. I initially opened an MR here but it is now closed: #585

Tracebacks (optional)

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Environment details (optional)

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Would you like to work on this issue?

Yes