fix num_utterances=-1 bug in generator_base.py

Author: Guangsen Wang

botsim/modules/generator/generator_base.py

@@ -187,7 +187,7 @@ def generate_goals(self,
 
         # if paraphrase:
         para_config = "_".join([str(x) for x in self.num_paraphrases_per_model])
-        if number_utterances > 0:
+        if isinstance(number_utterances, int) and number_utterances > 0:
             para_config = para_config + "_" + str(number_utterances)+"_utts"
         else:
             para_config = para_config + "_all_utts"