Adding I_ERROR into Correlated Errors

[Jalopfer]

The I_ERROR noisy gate was included in Stim. However, If I am not mistaken, it is currently not supported in Correlated Errors' instructions, e.g:

  circ.append_operation("CORRELATED_ERROR", ["X0", "I1"], 0.5).

I guess, adding It should be easy.

[Strilanc]

What's the use case you have in mind for this?

Currently pauli targets are defined by the regular expression !?[XYZ]\d+, so strictly speaking I1 is not a pauli target. It could be doable to allow CORRELATED_ERROR to be given qubit targets like 1... it's a bit janky compared to hiding it off in a dedicated jank instruction (such as I_ERROR). I think I'd be more inclined to allow I_ERROR to target paulis and measurement records.