Fix a bug in isometry.rs

[ShellyGarion]

Summary

After PR #12197 was merged, there is a bug with the following code:

num_qubits = 3
num_controls = 4
u = random_unitary(2 ** num_qubits)
ug = UnitaryGate(u)
cug = ug.control(num_controls)
cug 

The code is working for num_qubits = 1 or 2 but not for 3 or 4 or 5, and raises the following error:

/qiskit/circuit/library/generalized_gates/isometry.py:221, in Isometry._disentangle(self, circuit, q, diag, remaining_isometry, column_index, s)
    217 diagonal_mcg = self._append_mcg_up_to_diagonal(
    218     circuit, q, gate, control_labels, target_label
    219 )
    220 # apply the MCG to the remaining isometry
--> 221 v = isometry_rs.apply_multi_controlled_gate(v, control_labels, target_label, gate)
    222 # correct for the implementation "up to diagonal"
    223 diag_mcg_inverse = np.conj(diagonal_mcg).astype(complex, copy=False)

PanicException: assertion failed: j <= 1

Details and comments

The solution is to remove the assertion assert!(j <= 1) in crates/accelerate/src/isometry.rs:215:13

[qiskit-bot]

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

• @Eric-Arellano
• @Qiskit/terra-core
• @kevinhartman
• @mtreinish

[coveralls]

Pull Request Test Coverage Report for Build 9273316343

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

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.003%) to 89.586%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	7	92.37%
crates/qasm2/src/parse.rs	18	96.69%

Totals
Change from base Build 9224776217:	-0.003%
Covered Lines:	62317
Relevant Lines:	69561

---

💛 - Coveralls

[mtreinish]

This LGTM thanks for catching and fixing this. TBH, at the time when I added that assert I can't remember what I thought I was guarding against, but do remember explicitly thinking it was needed. But looking at it in retrospect it's clearly not correct for more than 2 target qubits so I'm not sure what I was thinking (I probably had tunnel vision trying to get the tests to pass).

Just one minor nit in the release notes but otherwise this is ready to go.