code: def _add_sphere(ax):
      
      (u, v) = np.mgrid[0:2 * np.pi:20j, 0:np.pi:10j]
      x = np.cos(u) * np.sin(v)
      y = np.sin(u) * np.sin(v)
      z = np.cos(v)
      ax.plot_wireframe(x, y, z, color='grey', linewidth=0.2)
      return ax
docs: _add_sphere(ax)
  
      Add a wireframe unit sphere onto matplotlib 3D axes
  
      Args:
          ax - matplotlib 3D axes object
  
      Returns:
          updated matplotlib 3D axes