Dynamical Decoupling pass with pulse alignment

[nkanazawa1989]

Summary

In recent IBM Quantum systems all pulses should start at the multiple value of 16 dt which is called pulse alignment constraint. This PR adds alignment constraint to the DD pass so that circuit with the DD sequence can be played on the actual hardware. The pass is initialized with pulse_alignment which defaults to 1, but you can give the value imposed by the backend. You can find this value in backend.configuration().timing_constraints.

Details and comments

There are several internal changes.

• DD pass is now subclass of BasePaddingpass. This means DD is a drop-in-replacement of PadDelay pass.
• Now _pad method is called with t0 and t1 instead of interval. This is for future extendability for the conditioning.
• Extra slack is now distributed to the edges of the sequence rather than to the interval in the middle.
• The import path of DD pass is upgraded (but still under the same place in transpiler.passes.

[coveralls]

Pull Request Test Coverage Report for Build 2022583134

• 130 of 138 (94.2%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 83.525%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/scheduling/base_padding.py	22	24	91.67%
qiskit/transpiler/passes/scheduling/dynamical_decoupling.py	92	98	93.88%

Totals
Change from base Build 2018409982:	0.007%
Covered Lines:	52568
Relevant Lines:	62937

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💛 - Coveralls

[ajavadia]

looks good!