Fix parameter handling for NLocal(..., flatten=True) and standard gates

[Cryoris]

Summary

Fixes #13478.

The fast-path for parameterizing standard gates caused an inconsistent state of the circuit, which is now fixed by using the explicit circuit._append_standard_gate method.

Details and comments

I'm not 100% sure what exactly the inconsistency was. Another way to fix this issue was to have the Rust-side assign_parameter_inner to explicitly re-assign the params attribute of the cached Python gate. So I'm assuming the old path didn't correctly register the gate in the cache, but I couldn't pin point the exact location in the code. If someone has more insight that would be great to know 🙂

[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 12034807511

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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

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.002%) to 88.934%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/parse.rs	6	97.15%
crates/qasm2/src/lex.rs	7	91.48%

Totals
Change from base Build 11977726745:	-0.002%
Covered Lines:	79403
Relevant Lines:	89283

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💛 - Coveralls

[mtreinish]

I don't think this actually fixes the issue. This is sidestepping the underlying bug by changing the construction of the n local circuit to not initially populate the py gate cache anymore. This means that the params in the cached python gate objects and the what's stored in circuit data/circuit instruction/packed instruction don't go out of sync because there isn't anything cached yet. If you updated the test to read every gate object from the circuit before assigning the params I think this would fail in the same way reported in the issue.

To fix this fully I'm pretty sure you need to update: https://github.com/Qiskit/qiskit/blob/main/crates/circuit/src/circuit_data.rs#L1382-L1389 to update the cached py gate, if the cache has been set.

[mtreinish]

LGTM now, this looks the correct fix, thanks for updating.