run_classifier.py improvements
Hi !

This PR contains multiple improvements to the `run_classifier.py` file. 

The changes are:
- removing trailing whitespaces ([PEP 8](https://www.python.org/dev/peps/pep-0008/)),
- simplifying a bit the data processing code, in particular tensor formatting,
- fixing issue #83 by adapting the value of the `num_labels` argument of `BertForSequenceClassification.from_pretrained` to the dataset being used. 
Neat!