Denormalize `x` with `mean` and `std`.

def denormalize(x:TensorImage, mean:FloatTensor,std:FloatTensor, do_x:bool=True)->TensorImage:
    "Denormalize `x` with `mean` and `std`."
    return x.cpu().float()*std[...,None,None] + mean[...,None,None] if do_x else x.cpu()