Fix an error in qasm3 exporter when operating on unitary gates

[gadial]

Summary

Fixes a bug causing the qasm3 exporter to crash on circuits containing a unitary gate.

Fixes #13362.

Details and comments

The bug results from the params field of the unitary operation being handled incorrectly, due to it being a somewhat nonstandard field (as opposed to the params field of u gate, for instance, which is used differently).

This problem was already handled in the qasm2 exporter by a call to _qasm2_decomposition in the operation, if present. This PR integrates the same solution in the qasm3 exporter, renaming the method to _qasm_decomposition.

[qiskit-bot]

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

• @Cryoris
• @Qiskit/terra-core
• @ajavadia

[coveralls]

Pull Request Test Coverage Report for Build 12769245443

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 9 of 9 (100.0%) changed or added relevant lines in 4 files are covered.
• 839 unchanged lines in 53 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.921%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/converters.rs	1	95.65%
crates/accelerate/src/sabre/heuristic.rs	1	32.87%
qiskit/circuit/library/iqp.py	1	96.15%
qiskit/transpiler/passes/layout/vf2_post_layout.py	1	85.71%
qiskit/circuit/library/boolean_logic/quantum_or.py	1	98.08%
qiskit/circuit/parameter.py	1	98.36%
qiskit/circuit/library/boolean_logic/quantum_and.py	1	97.96%
qiskit/circuit/library/boolean_logic/inner_product.py	1	96.0%
qiskit/synthesis/arithmetic/multipliers/rg_qft_multiplier.py	1	95.83%
crates/qasm2/src/expr.rs	1	94.02%

Totals
Change from base Build 12670558519:	-0.02%
Covered Lines:	79440
Relevant Lines:	89338

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💛 - Coveralls

[jakelishman]

I think this is a fine way to go about solving the problem, thanks. At first I was a bit nervous because the gate models of OQ2 and OQ3 aren't necessarily identical (OQ3 has scope to be more permissive), but in practice I doubt it'll come up any time soon, if ever. If it does, we'll deal with it then.