Change to Default CUDA GPU

[liordvir]

In prox_diffpir_poisson.py, line 115:

device = torch.device('cuda:2' if torch.cuda.is_available() else 'cpu')

the default CUDA GPU the script searches for is the 3rd available one, else it uses CPU.
This line will cause an error if user has 1 or 2 GPUs available, because torch.cuda.is_available() is True, but cuda:2 does not exist and the script tries to access it anyway.
I believe this should be changed to just 'cuda', so it defaults to user's first available GPU. (As many users probably only have at most one):

device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')

Thank you.