Set num_qubits kwarg when building UnitaryGate from Rust

[mtreinish]

Summary

In the accelerate crate we currently have two places that are building UnitaryGate, the quantum_volume() function that builds a quantum volume model circuit and the Split2QUnitaries transpiler pass. Currently this can only be done by calling Python (until #13272 is implemented) and there was a potential optimization we could make to specify the number of qubits as an argument to the Python constructor. This skipss the need for a few python operations to compute the number of qubits from the size of the matrix. These operations are not exceedingly slow, but as these Python space constructors are often the bottleneck so it should help runtime performance of these two functions.

Details and comments

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 11186666538

Details

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.01%) to 88.885%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/accelerate/src/two_qubit_decompose.rs	1	91.45%
crates/qasm2/src/lex.rs	4	92.23%

Totals
Change from base Build 11181214888:	0.01%
Covered Lines:	74473
Relevant Lines:	83786

---

💛 - Coveralls

[ElePT]

LGTM! One question out of curiosity, did you get to test how much faster the creation is when you give the number of qubits upfront?

[mtreinish]

I did a quick test just now with quantum_volume(), and it's not really measurable outside of the error margin on the benchmarks I've run. Realistically all we're saving here is skipping:

math.log2(matrix.shape[0])

for each unitary which isn't that expensive.