cherry-pick data_feeder bug fix to 1.6 (#20840)

* test=document_fix

* test=release/1.6

python/paddle/fluid/data_feeder.py

@@ -27,13 +27,7 @@ __all__ = ['DataFeeder']
 
 
 def convert_dtype(dtype):
-    if isinstance(dtype, str):
-        if dtype in [
-                'bool', 'float16', 'float32', 'float64', 'int8', 'int16',
-                'int32', 'int64', 'uint8'
-        ]:
-            return dtype
-    else:
+    if isinstance(dtype, core.VarDesc.VarType):
         if dtype == core.VarDesc.VarType.BOOL:
             return 'bool'
         elif dtype == core.VarDesc.VarType.FP16:
@@ -52,6 +46,19 @@ def convert_dtype(dtype):
             return 'int64'
         elif dtype == core.VarDesc.VarType.UINT8:
             return 'uint8'
+    else:
+        if dtype in [
+                'bool', 'float16', 'float32', 'float64', 'int8', 'int16',
+                'int32', 'int64', 'uint8', u'bool', u'float16', u'float32',
+                u'float64', u'int8', u'int16', u'int32', u'int64', u'uint8'
+        ]:
+            # this code is a little bit dangerous, since error could happen
+            # when casting no-asci code to str in python2.
+            # but since the set itself is limited, so currently, it is good.
+            # however, jointly supporting python2 and python3, (as well as python4 maybe)
+            # may still be a long-lasting problem.
+            return str(dtype)
+
     raise ValueError(
         "dtype must be any of [bool, float16, float32, float64, int8, int16, "
         "int32, int64, uint8]")