GeneralizedProximalGradient not working with CuPy

[mrava87]

Problem

GeneralizedProximalGradient is currently broken with CuPy when show=True because `x[0]/x[0, 0] are not moved to numpy:

pyproximal/pyproximal/optimization/primal.py

Line 885 in 5698eff

x[0] if x.ndim == 1 else x[0, 0],

Solution

Use np.real(to_numpy(x[0])) if x.ndim == 1 else np.real(to_numpy(x[0, 0])) instead.