Skip zero-qubit global_phase instructions during circuit separation

[TSS99]

Summary

separate_circuit() (and therefore partition_problem()) raised an AssertionError when the input circuit contained a zero-qubit global_phase instruction.

Details

A global_phase instruction acts on no qubits, so it spans no partition and cannot be assigned to a subcircuit. It is now skipped while assigning operations to subcircuits. Global phase does not affect the expectation values the cutting workflow reconstructs, so dropping it during separation is safe.

Tests

• pytest test/utils/test_transforms.py test/test_cutting_decomposition.py
• ruff check and black --check on the changed files

Fixes #762

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[garrison]

LGTM. Thank you for this contribution.

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