im2txt: make python3 compatible adding lt and eq

__cmp__ is deprecated on python3, so it fails to compare class Caption on python3

im2txt/im2txt/inference_utils/caption_generator.py

@@ -54,6 +54,16 @@ def __cmp__(self, other):
       return -1
     else:
       return 1
+
+  # for version 3 compatibility (__cmp__ is deprecated)
+  def __lt__(self, other):
+    assert isinstance(other, Caption)
+    return self.score < other.score
+
+  # also for version 3 compatibility
+  def __eq__(self, other):
+    assert isinstance(other, Caption)
+    return self.score == other.score
 
 
 class TopN(object):