Check data format consistency in data_feeder (#8417)

python/paddle/v2/fluid/data_feeder.py

@@ -107,6 +107,9 @@ class DataFeeder(object):
                     dtype=dtype))
 
         for each_sample in iterable:
+            assert len(each_sample) == len(converter), (
+                "The number of fields in data (%s) does not match " +
+                "len(feed_list) (%s)") % (len(each_sample), len(converter))
             for each_converter, each_slot in six.zip(converter, each_sample):
                 each_converter.feed(each_slot)
         ret_dict = {}