load_and_cache_examples crashes on windows
## 🐛 Bug

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Model I am using ALBERT:

Language I am using the model on (English):

The problem arise when using:
[examples/run_squad.py] the official example scripts: (run evaluation for offline model)

It crashes in "load_and_cache_examples" for paths with windows format. It's due to split being done using ('/'). For windows, it needs to be ('\\\\')

The tasks I am working on is:
SQUaD

## To Reproduce

Steps to reproduce the behavior:

1. Get one model cached into a local folder by running Evaluation for SQUAD 2.0 using run_squad.py. This will load online model to the system
2. Run evaluation again with model_name_or_path needs to be a local relative path with "..\\" in it.

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## Expected behavior

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## Environment

* OS: Windows
* Python version: Python 3.6
* PyTorch version: torch 1.3.1
* PyTorch Transformers version (or branch): Latest

## Additional context

Not needed

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.
