Mat.flatten does not return a Mat.mat

[jfeser]

Mat.flatten returns a one-dimensional ndarray, so using Mat operations on it fails with a "wrong number of indices" error.

[tachukao]

Hi @jfeser , I think the idea of flatten is to mimic numpy.flatten, which returns a 1D ndarray.

Implementation-wise, flatten is just a simple wrapper around. Maybe you can try Mat.(reshape x [|1; numel x|])? Would that be acceptable?