def __init__(self, density_matrix: np.ndarray, buffer: list[np.ndarray] | None = None):
        """Initializes the object with the inputs.

        This initializer creates the buffer if necessary.

        Args:
            density_matrix: The density matrix, must be correctly formatted. The data is not
                checked for validity here due to performance concerns.
            buffer: Optional, must be length 3 and same shape as the density matrix. If not
                provided, a buffer will be created automatically.
        Raises:
            ValueError: If the array is not the shape of a density matrix.
        """
        self._density_matrix = density_matrix
        if buffer is None:
            buffer = [np.empty_like(density_matrix) for _ in range(3)]
        self._buffer = buffer
        if len(density_matrix.shape) % 2 != 0:  # pragma: no cover
            raise ValueError('The dimension of target_tensor is not divisible by 2.')
        self._qid_shape = density_matrix.shape[: len(density_matrix.shape) // 2]
Initializes the object with the inputs.

        This initializer creates the buffer if necessary.

        Args:
            density_matrix: The density matrix, must be correctly formatted. The data is not
                checked for validity here due to performance concerns.
            buffer: Optional, must be length 3 and same shape as the density matrix. If not
                provided, a buffer will be created automatically.
        Raises:
            ValueError: If the array is not the shape of a density matrix.
        
cirq-core/cirq/sim/density_matrix_simulation_state.py