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_tests.rs

@@ -0,0 +1,6 @@
+/// Regression test for pth files not loading on Windows.
+#[test]
+fn test_pth() {   
+    let tensors = candle_core::pickle::PthTensors::new("tests/test.pt").unwrap();
+    tensors.get("test").unwrap().unwrap();
+}