feat: ToCupy operator

[mrava87]

Addresses #620

[mrava87]

@cako I'd like to hear your opinion on this before implementing the changes to all the other solvers.

Basically when I started to work on this I wasn't sure how much effort it would be required to have these sort of mixed CPU-GPU operators (and inverse problems). It turned out to be easier than expected, but it required some modifications to the solvers (hidden from the user) and the introduction of a new optional parameter for the stacking operators to be able to handle case 4 in #620. To handle case 3, it is instead simply required to have this new ToCupy operator...

Examples: https://github.com/PyLops/pylops_notebooks/blob/master/developement-cupy/ToCupy.ipynb

[cako]

Looks really good! The only part I'd say could be improved is the documentation.

[mrava87]

@cako what about now? I added 3 graphics for end-to-end GPU, and the 2 mixed scenarios 😄

[cako]

Looks awesome!! Super clear now!