Replace functional.sigmoid with torch.sigmoid (#3307)

Removes deprecation warning

Author: fmassa

torchvision/models/detection/rpn.py

@@ -252,7 +252,7 @@ def filter_proposals(self, proposals, objectness, image_shapes, num_anchors_per_
         levels = levels[batch_idx, top_n_idx]
         proposals = proposals[batch_idx, top_n_idx]
 
-        objectness_prob = F.sigmoid(objectness)
+        objectness_prob = torch.sigmoid(objectness)
 
         final_boxes = []
         final_scores = []