For the first error:

AttributeError: ‘RandomRotation’ object has no attribute ‘tfms’

It’s a typo . -> , :
tfms.Compose([tfms.RandomRotation(180), tfms.ToTensor()])

Also your __init__ functione have 2 parameters (self is used by python and represent the object instance) root_dir and transforms.
On the line full_dataset = Dataloader_img(...) you provides 3 parameters, the directory, the nii file and transform object. At least you need to removes the nii file.
For your second error it’s normal you tried to use a directory as input for nib.load(cwd) replace this with a .nii file and you should be fine.