grayscale is now inferred from options if it is not explicitly specified

[IljaManakov]

Fixes #46

[andreasoie]

Have you even tested this? Because I don't think it'll work since:

File "../../CUT/models/cut_model.py", line 151, in forward                                    │
    self.real = torch.cat((self.real_A, self.real_B), dim=0) if self.opt.nce_idt and self.opt.isTrain els│
e self.real_A                                                                                            │
RuntimeError: Sizes of tensors must match except in dimension 0. Expected size 3 but got size 1 for tenso│
r number 1 in the list.