Make response size computation py3 compatible

Reviewed By: ashwinb

Differential Revision: D10218305

fbshipit-source-id: d9d285959b1e4f5c0d38f080d6ce8c260b1959e0

detectron/utils/io.py

@@ -136,7 +136,10 @@ def download_url(
     https://stackoverflow.com/questions/2028517/python-urllib2-progress-hook
     """
     response = urllib.request.urlopen(url)
-    total_size = response.info().getheader('Content-Length').strip()
+    if six.PY2:
+        total_size = response.info().getheader('Content-Length').strip()
+    else:
+        total_size = response.info().get('Content-Length').strip()
     total_size = int(total_size)
     bytes_so_far = 0