Add get_dt_from_backend func to the builder command

[to24toro]

Summary

Added a new buillder command get_dt_from_backend to builder.py.
This returns dt differently based with the type of a backend class.

Details and comments

Need to review after #10150.

[qiskit-bot]

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

• @Qiskit/terra-core
• @ikkoham
• @nkanazawa1989

[coveralls]

Pull Request Test Coverage Report for Build 5542245283

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 11 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.004%) to 86.004%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	80.0%
crates/qasm2/src/lex.rs	3	91.14%
crates/qasm2/src/parse.rs	6	97.58%

Totals
Change from base Build 5541406727:	0.004%
Covered Lines:	71753
Relevant Lines:	83430

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💛 - Coveralls

[TsafrirA]

LGTM.

[nkanazawa1989]

Thanks @to24toro , I think this simplifies internals of logic requiring backend dt. However, I have a suggestion for different approach.

Note that the pulse builder already supports commands seconds_to_samples and samples_to_seconds, and end user doesn't require the dt information other than converting time unit. This makes me think get_dt_from_backend doesn't need to be a builder command. Instead, you can implement a method to return backend dt directly in the _PulseBuilder class, e.g. _PulseBuilder.time_resolution, and let seconds_to_samples and samples_to_seconds access this method through _active_builder function?

https://github.com/Qiskit/qiskit-terra/blob/e9f8b7c50968501e019d0cb426676ac606eb5a10/qiskit/pulse/builder.py#L1021

Since _PulseBuilder is a private class, its method is much more maintainable compared with the public builder command.

[to24toro]

@nkanazawa1989
Can I remove the release note because my change only affects the private class _PulseBuilder and has minimal impact on users?

[nkanazawa1989]

Please remove this note because this is now change in the internals (the builder class is protected) so there is no need for end user communication.

[to24toro]

I fixed at 8effc99.

[nkanazawa1989]

Thanks this makes code much readable and clean.