fix: add batch size check in loading data process

Author: chenwhql

python/paddle/batch.py

@@ -40,4 +40,9 @@ def batch_reader():
         if drop_last == False and len(b) != 0:
             yield b
 
+    # Batch size check
+    if batch_size <= 0:
+        raise ValueError("batch_size should be a positive integeral value, "
+                         "but got batch_size={}".format(batch_size))
+
     return batch_reader