Label handling commit breaks the imdb finetuning script

Thomas, thanks for sharing this code! I noticed that commit 8d9c2371fc37ba8958f174501a9afb91a7ef7a06 seems to have broken the default functioning of the classification finetuning scripts - in the previous version there seems to have been a key called `'labels'` associated with the imdb and trec dictionaries, but in `finetuning_train.py` [this line](https://github.com/huggingface/naacl_transfer_learning_tutorial/blob/69031641946979f0ce1a0961a6b50e290f3de434/utils.py#L127) still references the now deleted key.

I updated the line to just use `DATASETS_LABELS_URL['imdb']['test']` as intended, but then it seems that the S3 bucket doesn't have the IMDB test file.

See below:
```
file_path = "https://s3.amazonaws.com/datasets.huggingface.co/imdb/test.labels.txt"
label_file = cached_path(file_path)
with open(label_file, "r", encoding="utf-8") as f:
all_lines = f.readlines()
print(all_lines[:5])
```
Gives:
```
['\n', 'NoSuchKeyThe specified key does not exist.imdb/test.labels.txt3D9E7C511167A0FBRiidOcrHfFaqxW9tmUXRppE/G3lsYoCZcq+uaYDi2yPPoe8mv/Og6PMuUncwk+B53tGsvcCZMWk=']
```

Does the test file for IMDB still exist with this name? This doesn't seem to be an issue with TREC.