check for errors in SemSegFPNHead

Summary: close facebookresearch#3316

Reviewed By: wat3rBro

Differential Revision: D30062593

fbshipit-source-id: e8b614c21f62012e081f425dd9df850179f1f2a7

Author: ppwwyyxx

detectron2/modeling/meta_arch/semantic_seg.py

@@ -163,6 +163,8 @@ def __init__(
         """
         super().__init__()
         input_shape = sorted(input_shape.items(), key=lambda x: x[1].stride)
+        if not len(input_shape):
+            raise ValueError("SemSegFPNHead(input_shape=) cannot be empty!")
         self.in_features = [k for k, v in input_shape]
         feature_strides = [v.stride for k, v in input_shape]
         feature_channels = [v.channels for k, v in input_shape]