Improve error message for unrecognized vocab type for text problems.

PiperOrigin-RevId: 202169649

Author: urvashik

tensor2tensor/data_generators/imdb.py

@@ -101,5 +101,6 @@ def generate_samples(self, data_dir, tmp_dir, dataset_split):
 class SentimentIMDBCharacters(SentimentIMDB):
   """IMDB Sentiment classification, character level."""
 
+  @property
   def vocab_type(self):
     return text_problems.VocabType.CHARACTER

tensor2tensor/data_generators/text_problems.py

@@ -226,7 +226,8 @@ def get_or_create_vocab(self, data_dir, tmp_dir, force_get=False):
       encoder = text_encoder.TokenTextEncoder(vocab_filename,
                                               replace_oov=self.oov_token)
     else:
-      raise ValueError("Unrecognized VocabType")
+      raise ValueError(
+          "Unrecognized VocabType: %s" % str(self.vocab_type))
     return encoder
 
   def _maybe_pack_examples(self, generator):