corrected UV coordinates flip for torso

Summary:
Fixes flip transform for body parts that do not change labels under flip:
- old behavior: UV coordinates unchanged
- new behavior: UV coordinates flipped

Reviewed By: MarcSzafraniec

Differential Revision: D19327508

fbshipit-source-id: 7747e56b2746984452d13db9a7f6536034eba114

Author: vkhalidov

projects/DensePose/densepose/structures.py

@@ -152,14 +152,15 @@ def _transform_pts(self, transforms, dp_transform_data):
             self.x = self.segm.size(1) - self.x
             self._flip_iuv_semantics(dp_transform_data)
 
-    def _flip_iuv_semantics(self, dp_transform_data):
+    def _flip_iuv_semantics(self, dp_transform_data: DensePoseTransformData) -> None:
         i_old = self.i.clone()
         uv_symmetries = dp_transform_data.uv_symmetries
         pt_label_symmetries = dp_transform_data.point_label_symmetries
         for i in range(self.N_PART_LABELS):
-            if pt_label_symmetries[i + 1] != i + 1:
-                annot_indices_i = i_old == i + 1
-                self.i[annot_indices_i] = pt_label_symmetries[i + 1]
+            if i + 1 in i_old:
+                annot_indices_i = (i_old == i + 1)
+                if pt_label_symmetries[i + 1] != i + 1:
+                    self.i[annot_indices_i] = pt_label_symmetries[i + 1]
                 u_loc = (self.u[annot_indices_i] * 255).long()
                 v_loc = (self.v[annot_indices_i] * 255).long()
                 self.u[annot_indices_i] = uv_symmetries["U_transforms"][i][v_loc, u_loc]