Swap unsafe_chunk() for chunk() (#7362)

Eliminates all unsafe function in YOLOv5 out of an abundance of caution.

Author: glenn-jocher

utils/loss.py

@@ -220,7 +220,7 @@ def build_targets(self, p, targets):
                 offsets = 0
 
             # Define
-            bc, gxy, gwh, a = t.unsafe_chunk(4, dim=1)  # (image, class), grid xy, grid wh, anchors
+            bc, gxy, gwh, a = t.chunk(4, 1)  # (image, class), grid xy, grid wh, anchors
             a, (b, c) = a.long().view(-1), bc.long().T  # anchors, image, class
             gij = (gxy - offsets).long()
             gi, gj = gij.T  # grid indices