fix to pool with scale 1

Summary: Small fix to pool at scale 1 in ROIPooler

Reviewed By: ppwwyyxx

Differential Revision: D22809193

fbshipit-source-id: 2512b47354a85bca6590496173bd6706e274e3cc

Author: gkioxari

detectron2/modeling/poolers.py

@@ -171,7 +171,7 @@ def __init__(
         assert (
             len(scales) == self.max_level - self.min_level + 1
         ), "[ROIPooler] Sizes of input featuremaps do not form a pyramid!"
-        assert 0 < self.min_level and self.min_level <= self.max_level
+        assert 0 <= self.min_level and self.min_level <= self.max_level
         self.canonical_level = canonical_level
         assert canonical_box_size > 0
         self.canonical_box_size = canonical_box_size