Fix idle qubit detection to ignore barriers

[raunakkumarsingh]

Fixes #621
Check for Barriers:
Added if instruction.operation.name == 'barrier': continue to skip over barrier operations during the idle wire detection process.
Iterate Through Instructions:

For each instruction, if it is not a barrier, it iterates through the qubits involved in the instruction and removes those qubit indices from the idle_wires set.
Filter Qubit Subsets:

The qubit_subsets list is filtered to remove any subsets that consist of only one qubit, and that qubit is in the idle_wires set.

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[raunakkumarsingh]

Added if instruction.operation.name == 'barrier': continue to skip over barrier operations during the idle wire detection process.

[coveralls]

Pull Request Test Coverage Report for Build 10002802415

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Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 9983091228:	0.0%
Covered Lines:	2412
Relevant Lines:	2412

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💛 - Coveralls

[garrison]

It'd be nice to have an explicit test for this behavior, too.

[garrison]

The lint failure can be fixed by merging main into this branch once #640 has been merged to main.

[garrison]

@raunakkumarsingh, thank you for this contribution. Do you plan to add a test and merge main into this branch, or should I do it?

[raunakkumarsingh]

I have added tests for the barrier Ignore and rebased my branch.

[garrison]

Thank you. Please run tox -e style and make sure that tox -e lint passes.

[raunakkumarsingh]

corrected lint

[garrison]

Actually, I believe this test does not actually check that the relevant bug is fixed.

The idle wire detection code only runs when partition_labels is not provided (or is None). In this case, the partition labels are determined automatically from the connectivity of the circuit. #594 improved on this code so that idle wires are removed from the circuit during separation, but the problem is that any qubit with a barrier (e.g., if a barrier is placed over all qubits, even the idle ones) will be falsely detected as an active qubit. Your change should indeed fix #621 but the test does not ensure that it is fixed. The test is actually almost testing the check desired in #626, but not quite.