Fix camera clone() with torch.save

Summary:
User reported that cloned cameras fail to save. The error with latest PyTorch is

```
pickle.PicklingError: Can't pickle ~T_destination: attribute lookup T_destination on torch.nn.modules.module failed
```

This fixes it.

Reviewed By: btgraham

Differential Revision: D39692258

fbshipit-source-id: 75bbf3b8dfa0023dc28bf7d4cc253ca96e46a64d

tests/test_cameras.py

@@ -31,6 +31,7 @@
 # SOFTWARE.
 
 import math
+import pickle
 import typing
 import unittest
 from itertools import product
@@ -1333,6 +1334,11 @@ class TestPerspectiveProjection(TestCaseMixin, unittest.TestCase):
         # Check in_ndc is handled correctly
         self.assertEqual(cam._in_ndc, c0._in_ndc)
 
+    def test_clone_picklable(self):
+        camera = PerspectiveCameras()
+        pickle.dumps(camera)
+        pickle.dumps(camera.clone())
+
 
 ############################################################
 #                FishEye Camera                        #