Verifies the density_matrix_rep is valid and converts it to ndarray form.

This method is used to support passing a matrix, a state vector, or a computational basis state as a representation of a state.

density_matrix_rep If a numpy array, if it is of rank 2 (a matrix), then this is the density matrix. If it is a numpy array of rank 1 (a vector) then this is a state vector. If this is an int, then this is the computation basis state.
num_qubits The number of qubits for the density matrix. The density_matrix_rep must be valid for this number of qubits.
qid_shape The qid shape of the state vector. Specify this argument when using qudits.
dtype The numpy dtype of the density matrix, will be used when creating the state for a computational basis state (int), or validated against if density_matrix_rep is a numpy array.
atol Numerical tolerance for verifying density matrix properties.

A numpy matrix corresponding to the density matrix on the given number of qubits. Note that this matrix may share memory with the input density_matrix_rep.

ValueError if the density_matrix_rep is not valid.