Feature Request: Option to run on CPU

[jraufeisen]

Thanks for this awesome implementation of CAMERAS!
I was experimenting with it on a non-GPU machine first and found that by default this library assumes that CUDA is installed. But with a few tweaks it's possible to run it on CPU only.

Could we add functionality for that?

An intuitive implementation could be to check inside the constructor of CAMERAS(<model>, ...) which device the given model lives in. Then all existing calls to .cuda() can be replaced with .to(<device>).

What do you think?