Add debug info

Author: velconia

python/paddle/reader/decorator.py

@@ -325,6 +325,14 @@ def xreader():
             else:
                 yield sample
 
+        assert finish == process_num
+        for w in workers:
+            print('join worker: ', w)
+            print('join worker: ', w.is_alive())
+            w.join()
+            print('join worker: ', w.is_alive())
+        del workers
+
     return xreader