support voc format in fcos & ttfnet (#2875)

ppdet/data/transform/batch_operators.py

@@ -452,9 +452,10 @@ class Gt2FCOSTarget(BaseOperator):
                 sample['centerness{}'.format(lvl)] = np.reshape(
                     ctn_targets_by_level[lvl], newshape=[grid_h, grid_w, 1])
 
-            sample.pop('is_crowd')
-            sample.pop('gt_class')
-            sample.pop('gt_bbox')
+            sample.pop('is_crowd', None)
+            sample.pop('difficult', None)
+            sample.pop('gt_class', None)
+            sample.pop('gt_bbox', None)
         return samples
 
 
@@ -531,11 +532,11 @@ class Gt2TTFTarget(BaseOperator):
             sample['ttf_heatmap'] = heatmap
             sample['ttf_box_target'] = box_target
             sample['ttf_reg_weight'] = reg_weight
-            sample.pop('is_crowd')
-            sample.pop('gt_class')
-            sample.pop('gt_bbox')
-            if 'gt_score' in sample:
-                sample.pop('gt_score')
+            sample.pop('is_crowd', None)
+            sample.pop('difficult', None)
+            sample.pop('gt_class', None)
+            sample.pop('gt_bbox', None)
+            sample.pop('gt_score', None)
         return samples
 
     def draw_truncate_gaussian(self, heatmap, center, h_radius, w_radius):