chamfer test consistency

Summary:
Modify test_chamfer for more robustness. Avoid empty pointclouds, including where point_reduction is mean, for which we currently return nan (*), and so that we aren't looking at an empty gradient. Make sure we aren't using padding as points in the homogenous cases in the tests, which will lead to a tie between closest points and therefore a potential instability in the gradient - see https://github.com/pytorch/pytorch/issues/35699.

(*) This doesn't attempt to fix the nan.

Reviewed By: nikhilaravi, gkioxari

Differential Revision: D21157322

fbshipit-source-id: a609e84e25a24379c8928ff645d587552526e4af

tests/common_testing.py

@@ -123,4 +123,8 @@ class TestCaseMixin(unittest.TestCase):
             input, other, rtol=rtol, atol=atol, equal_nan=equal_nan
         )
 
+        if not close and msg is None:
+            max_diff = backend.abs(input - other).max()
+            self.fail(f"Not close. max diff {max_diff}.")
+
         self.assertTrue(close, msg)