adding absolute imports to gpt2, openai and transfo-xl

pytorch_pretrained_bert/modeling_openai.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 """PyTorch OpenAI GPT model."""
 
+from __future__ import absolute_import, division, print_function, unicode_literals
+
 import collections
 import copy
 import json