Construct subclasses of InstructionDurations from backend

[dieris]

Summary

Modify construction of InstructionDurations.from_backend to return an instance of any subclass, as in the use model for dynamic circuits on qiskit-ibm-provider

Details and comments

In the current behavior DynamicInstructionDurations.from_backend() (a subclass of InstructionDurations) returns an instance of InstructionDurations (and corresponding durations) instead.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 7427911562

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 87.591%

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Totals
Change from base Build 7426524855:	0.003%
Covered Lines:	59475
Relevant Lines:	67901

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💛 - Coveralls

[jakelishman]

This seems straightforwards and safe, thanks. This class doesn't look to have been designed for subclassing (there's no contract for when it will call its own methods, so it's fragile to depend on), but it's a simple mostly-data class, so it's probably not too risky if others are subclassing it.

Can you add a one-line bugfix release note mentioning the change?

[dieris]

Actually, using the right constructor now causes an error on qiskit-ibm-provider, but I'll open an issue there 😄

[jakelishman]

That seems weird, unless InstructionDurations.from_backend is already broken for IBMBackend (which it could well be - I'm pretty sure from_backend assumes a BackendV1 and has never been updated). I'm fine for this PR update the call here, because really, it's pretty rare for constructors to be safe for virtual override - much like __init__ is near-always necessarily overridden by subclasses, I'd expect all other constructors to be as well.

[mtreinish]

@Mergifyio backport stable/0.46

[mergify]

backport stable/0.46

✅ Backports have been created

• #11727 Construct subclasses of InstructionDurations from backend (backport #11501) has been created for branch stable/0.46