handle ImportError exception when used from projects outside
The relative path that starts with . does not work when a file is used from an outside project. I added a safe code to handle the ImportError exception in this case, so I can use the source file without having to make local changes to it.
Just doing `from pytorch_pretrained_bert import BertTokenizer, BertModel` in your project doesn't work?

That's what they do in [AllenNLP](https://github.com/allenai/allennlp/blob/3f0953d19de3676ea82e642659fc96d90690e34d/allennlp/modules/token_embedders/bert_token_embedder.py#L14) or [flair](https://github.com/zalandoresearch/flair/blob/797c958d0e8c256531f2cea37508e7becb2026cb/flair/embeddings.py#L14)<|||||>it does work, however, I don't need the higher level classes for my model, so I am sub classing the OpenAIGPTPreTrainedModel and Block and they are not included into the __init__.py. Including them into the __init__.py also solves the issue with the importException:

from .modeling_openai import (OpenAIGPTConfig, OpenAIGPTModel,
OpenAIGPTLMHeadModel, OpenAIGPTDoubleHeadsModel, OpenAIGPTPreTrainedModel, Block, load_tf_weights_in_openai_gpt)
<|||||>I see. I think we can fix this the same way I did in the `bert` case by adding `from __future__ import absolute_import`. If you don't mind I'll do a quick PR on that.