Working GLUE Example for XLNet (STS-B)
`run_glue.py` is now a single script able to train BERT, XLNet and XLM on all GLUE tasks.

Example for XLNet:
```bash
CUDA_VISIBLE_DEVICES=0,1,2,3 python ./examples/run_glue.py   --do_train  --task_name=sts-b     --data_dir=${GLUE_DIR}/STS-B   --output_dir=./proc_data/sts-b-110   --max_seq_length=128   --per_gpu_eval_batch_size=8   --per_gpu_train_batch_size=8   --max_steps=1200  --model_name=xlnet-large-cased   --overwrite_output_dir   --overwrite_cache --warmup_steps=120
```
These hyper-parameters (same as the original one) give a pearsonr > 0.918.
