Handle unsimplified QubitOperator sparse terms

[marko1olo]

Fixes #880.

Summary

• simplify QubitOperator terms inside sparse conversion before building Kronecker factors
• apply the same simplification to get_linear_qubit_operator_diagonal()
• add regressions for repeated same-qubit Pauli factors and the symmetry_conserving_bravyi_kitaev reproducer from the issue

Tests

• PYTHONPATH=C:\hades\oss\openfermion-880\src C:\hades\oss\openfermion\.venv\Scripts\python.exe -m pytest -p no:cacheprovider src\openfermion\linalg\sparse_tools_test.py::SparseOperatorTest::test_qubit_operator_sparse_simplifies_repeated_qubit_factors src\openfermion\linalg\sparse_tools_test.py::SparseOperatorTest::test_get_sparse_operator_with_unsimplified_symmetry_conserving_bk src\openfermion\linalg\sparse_tools_test.py::SparseOperatorTest::test_get_linear_qubit_operator_diagonal_simplifies_repeated_qubit_factors -q
• PYTHONPATH=C:\hades\oss\openfermion-880\src C:\hades\oss\openfermion\.venv\Scripts\python.exe -m pytest -p no:cacheprovider src\openfermion\linalg\sparse_tools_test.py -q
• PYTHONPATH=C:\hades\oss\openfermion-880\src C:\hades\oss\openfermion\.venv\Scripts\python.exe -m pytest -p no:cacheprovider src\openfermion\transforms\opconversions\remove_symmetry_qubits_test.py -q
• C:\hades\oss\openfermion\.venv\Scripts\python.exe -m black --check --diff src\openfermion\linalg\sparse_tools.py
• C:\hades\oss\openfermion\.venv\Scripts\python.exe -m black --check --diff src\openfermion\linalg\sparse_tools_test.py
• PYTHONPYCACHEPREFIX=C:\tmp\pycache-openfermion-880 C:\hades\oss\openfermion\.venv\Scripts\python.exe -m compileall -q src\openfermion\linalg\sparse_tools.py src\openfermion\linalg\sparse_tools_test.py
• git diff --check

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[gemini-code-assist]

Code Review

This pull request introduces a helper function _iter_simplified_qubit_operator_terms to ensure that QubitOperator terms are simplified (resolving repeated qubit factors) when generating sparse operators or calculating diagonals. Unit tests have been added to verify this behavior. The reviewer noted that calling _simplify on every term introduces a significant performance overhead for large operators and suggested an optimization to bypass _simplify when terms are already simplified.

[gemini-code-assist]

Calling _simplify on every term of the QubitOperator introduces a significant performance overhead (due to sorting, dictionary lookups, and tuple allocations), especially for large operators with tens of thousands of terms. Since the vast majority of QubitOperator instances are already simplified, we can optimize this by first checking if the term is already simplified (i.e., indices are strictly increasing and there are no identity 'I' operators). If it is already simplified, we can yield it directly and bypass _simplify entirely.

def _iter_simplified_qubit_operator_terms(qubit_operator):
    """Yield terms after applying QubitOperator's Pauli simplification."""
    for qubit_term, coefficient in qubit_operator.terms.items():
        is_simplified = True
        prev_index = -1
        for index, action in qubit_term:
            if action == 'I' or index <= prev_index:
                is_simplified = False
                break
            prev_index = index

        if is_simplified:
            yield qubit_term, coefficient
        else:
            coefficient, qubit_term = qubit_operator._simplify(qubit_term, coefficient)
            yield qubit_term, coefficient