Installation example #2 fails: cannot import name 'glue_compute_metrics'
## 🐛 Bug

<!-- Important information -->

I am having issues with the official installation procedure, where running `python -m pytest -sv ./examples` fails with an opaque error message (below).

## To Reproduce

Steps to reproduce the behavior:

1. Create virtualenv
2. Install Pytorch (`pip install torch==1.2.0+cpu torchvision==0.4.0+cpu -f https://download.pytorch.org/whl/torch_stable.html`)
3. Install transformers (`pip install transformers`)
4. Install pytest (`pip install pytest`)
5. Run `python -m pytest -sv ./transformers/tests/`; no tests fail
6. Run `python -m pytest -sv ./examples/`; fails requiring tensorboardX
7. Install tensorboardX (`pip install tensorboardX`)
8. Run `python -m pytest -sv ./examples/`; fails with message:

```
==================================== ERRORS ====================================
__________________ ERROR collecting examples/test_examples.py __________________
ImportError while importing test module '/home/evancw/Projects/transformers/examples/test_examples.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
examples/test_examples.py:30: in <module>
    import run_glue
examples/run_glue.py:49: in <module>
    from transformers import glue_compute_metrics as compute_metrics
E   ImportError: cannot import name 'glue_compute_metrics'
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.64s ===============================
```

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## Expected behavior

<!-- A clear and concise description of what you expected to happen. -->

I would expect the tests to pass.

## Environment

* OS: Ubuntu 18.04.3 LTS
* Python version: 3.6.8
* PyTorch version: 1.2.0 + CPU
* PyTorch Transformers version (or branch): 2.0.0
* Using GPU ? No
* Distributed of parallel setup ? No
* Any other relevant information: System is completely clean before pip installs

## Additional context

Please let me know if there is any more information I can provide.

 @evanweissburg  Hi, i got the same error..have you found any solution???
 <|||||>Hello! I believe you must have sklearn installed in order to pass these tests. Please let me know if it doesn't work while having sklearn installed.<|||||>yeah got it .... i guess we need to run pip install -r ./examples/requirements.txt<|||||>Indeed!<|||||>Could we add this to the getting started documentation?

pip install -r ./examples/requirements.txt<|||||>Had the same issue. Probably a problem with sklearn. Installed with conda and it was fixed.