add type conversion for batch_size

Author: chenwhql

python/paddle/batch.py

@@ -41,6 +41,7 @@ def batch_reader():
             yield b
 
     # Batch size check
+    batch_size = int(batch_size)
     if batch_size <= 0:
         raise ValueError("batch_size should be a positive integeral value, "
                          "but got batch_size={}".format(batch_size))