[BUGFIX] Import missing module

models/unsupervised.py uses `pt` in line 37, but `pt` is undefined in the file.
I wonder why Python doesn't complain about this. Perhaps because unsupervised.py
is never run in isolation and `pt` is otherwise available in the namespace of
the example scripts that use unsupervised.py.

prototorch/models/unsupervised.py

@@ -3,6 +3,7 @@
 import logging
 import warnings
 
+import prototorch as pt
 import pytorch_lightning as pl
 import torch
 import torchmetrics