Deprecate Qobj in unitary_simulator.py

[1ucian0]

I also just realized that qiskit.providers.basicaer.UnitarySimulatorPy.run() (https://github.com/Qiskit/qiskit/blob/main/qiskit/providers/basicaer/unitary_simulator.py#L206) never deprecated a Qobj input and will still support it while the qasm simulator and statevector simulator will not. I still think we should move forward with your PR, but it's something we'll want to clean up in a subsequent PR.

Originally posted by @mtreinish in #10872 (review)

[rupeshknn]

Hi, I'd like to work on this. the description isn't very clear, though, could you help me with what needs to be done? Should I just deprecate Qobj input or remove this functionality?

[rupeshknn]

When a QasmQobj is supplied to QasmSimulatorPy.run or StatevectorSimulatorPy.run, the error raised is:

QiskitError: 'bad input to assemble() function; must be either circuits or schedules'

And when a QasmQobj is supplied to QasmSimulatorPy.run, it complains that there's no n_qubits property for QasmQobj rather than clearly stating that only circuits are supported. (as the assemble call is tucked under a conditional)

Since a Qobj input cannot be used anyway, I think it's better to remove Qobj support directly rather than deprecate it and remove it in a later release. What do you think @1ucian0 @mtreinish?

[rupeshknn]

Based on the conversation here with @1ucian0 I'll start with a deprecation PR to stable\0.46 and once that's done, I'll do a removal PR to main.