Vocabularly file not available for Squad predictions
There appears to be a bug in the way the vocabulary file is handled.

For example, if we execute [`run_squad.py`](https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/examples/run_squad.py) with `--do_train`, and set the `--output_dir` to `/tmp/debug_squad/`, we successfully build a model and the resulting model files (`bert_config.json` and `pytorch_model.bin`) are saved in the appropriate directory.

Then we execute [`run_squad.py`](https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/examples/run_squad.py) with `--do_predict`, and this time set `--bert_model` to `/tmp/debug_squad`, which according to [`modelling.py`](https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/pytorch_pretrained_bert/modeling.py) is all that is required (see the [`from_pretrained`](https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/pytorch_pretrained_bert/modeling.py#L532) method).

However, this raises a bug as the tokenizer cannot load a vocabulary file. If we inspect [`tokenizer.py`](https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/pytorch_pretrained_bert/tokenization.py#L138) we can see what happens: if you are using a pre-trained BERT model, it will look for the vocab file at a specific URL. If you input a directory as your `bert_model`, it assumes you have a file `vocab.txt` (`VOCAB_NAME`) in that same directory. It also appears to check the cache which may or may not be present.

We were able to fix this by simply downloading the appropriate vocab file for our base BERT model (e.g., `'bert-base-uncased': "https://s3.amazonaws.com/models.huggingface.co/bert/bert-base-uncased-vocab.txt"`), renaming it to `vocab.txt`, and placing it in `/tmp/debug_squad`, however it feels as though this should be better handled by the train/predict pipeline.
Indeed, this example could be improved. I would happy to welcome a PR on that.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
