Fix: pth files don't load on Windows (#1661)

* Don't treat zip path as OS path

* Add a test case

* Add code to generate test pth data

candle-core/tests/pth.py

@@ -0,0 +1,8 @@
+import torch
+from collections import OrderedDict
+
+# Write a trivial tensor to a pt file
+a= torch.tensor([[1,2,3,4], [5,6,7,8]])
+o = OrderedDict()
+o["test"] = a
+torch.save(o, "test.pt")