input: Write a python function to solve a PDE using scipy's ode solver.
  assert np.allclose(pde_solve(1, 0, 1, 0.1), np.exp(1), atol=1e-2)
  assert np.allclose(pde_solve(2, 0, 2, 0.1), np.exp(2)*2, atol=1e-2)
  assert np.allclose(pde_solve(0.5, 0, 3, 0.1), np.exp(3)*0.5, atol=1e-2)
output: import numpy as np
  from scipy.integrate import odeint
  
  def pde_solve(a, x0, t, dt):
      def model(y, t):
          return a * y
  
      y0 = np.exp(x0)
      t_points = np.arange(0, t + dt, dt)
      sol = odeint(model, y0, t_points)
      return sol[-1][0]