add BaseSegmentor import to segmentors/__init__.py (open-mmlab#495)

Author: Ziyi Wu

mmseg/models/segmentors/__init__.py

@@ -1,4 +1,5 @@
+from .base import BaseSegmentor
 from .cascade_encoder_decoder import CascadeEncoderDecoder
 from .encoder_decoder import EncoderDecoder
 
-__all__ = ['EncoderDecoder', 'CascadeEncoderDecoder']
+__all__ = ['BaseSegmentor', 'EncoderDecoder', 'CascadeEncoderDecoder']