Minor changes to backend import

import numpy as np

from scipy.sparse import diags

# Importing Qiskit libraries

from qiskit import QuantumCircuit, transpile

from qiskit import QuantumCircuit

from qiskit_aer import AerSimulator

from linear_solvers import NumPyLinearSolver, HHL

from linear_solvers.matrices.tridiagonal_toeplitz import TridiagonalToeplitz

# ============

# Select backend: Using different simulators (default in `linear_solvers` is statevector simulation)

from qiskit import Aer

# https://qiskit.org/documentation/tutorials/simulators/1_aer_provider.html

# run `Aer.backends()` to see simulators

backend = Aer.get_backend('aer_simulator_statevector')

backend = AerSimulator(method='statevector')

if args.gpu: backend.set_options(device='GPU')

backend.set_options(precision='single')