CR Hamiltonian experiment without cr_gate

[nkanazawa1989]

Summary

Bug fix for CrossResonanceHamiltonian experiment instantiation without cr gate.

Details and comments

self._cr_gate should be assigned before calling super init method since it calls set backend where we check the cr gate.

https://qiskit.slack.com/archives/C02CFNAKURE/p1651458517025279

[wshanks]

Looks good! I wonder if the different cases (with and without backend/gate) should be tested more systematically.

[nkanazawa1989]

Thanks Will, actually I found another bug with extra test 12faf53

self._dt, self._granularity, and self._cr_channel are initialized in the constructor but after the super init method call is accomplished. Since it internally populates these values when it's called with backend through self._set_backend method, these value SHOULD be initialized before the super init method call. Otherwise, populated values are overridden. It's really critical bug.

[wshanks]

Looks good.

I wonder though if you should more systematically have four tests, with/without backend and with/without cr_gate that all make sure circuits works without error. I guess you cover all cases except no backend with cr_gate currently.

[nkanazawa1989]

Thanks,

no backend with cr_gate currently

This is tested in test_instance_with_backend_without_cr_gate. When we set the backend later, this is identical to setting them together.