Fix Type Error in Conditional Logic (#4294)

Summary:
The old logic was asking for `D`'s shape, which indicated that it should be a NumPy array. However, if you check the truth value of a NumPy array, you get an error.

The code is checking for the existence of `D`. The most direct way to handle that is to make sure that it isn't `None`.

Pull Request resolved: #4294

Reviewed By: junjieqi

Differential Revision: D72972114

Pulled By: mnorris11

fbshipit-source-id: 4ffabcccfd7418f625a379b8c3684d83bf57cac7

Author: mjs2600

faiss/python/class_wrappers.py

@@ -1038,7 +1038,7 @@ def replacement_vt_train(self, x):
 
 def handle_AutoTuneCriterion(the_class):
     def replacement_set_groundtruth(self, D, I):
-        if D:
+        if D is not None:
             assert I.shape == D.shape
         self.nq, self.gt_nnn = I.shape
         self.set_groundtruth_c(