Revert "Remove the default None for data_path, which is a required param."

This reverts commit 6d829caa.

tutorials/rnn/ptb/reader.py

@@ -52,7 +52,7 @@ def _file_to_word_ids(filename, word_to_id):
   return [word_to_id[word] for word in data if word in word_to_id]
 
 
-def ptb_raw_data(data_path):
+def ptb_raw_data(data_path=None):
   """Load PTB raw data from data directory "data_path".
 
   Reads PTB text files, converts strings to integer ids,