Update run_xnli.py to use Datasets library (#9829)

* remove xnli_compute_metrics, add load_dataset, load_metric, set_seed,metric.compute,load_metric

* fix

* fix

* fix

* push

* fix

* everything works

* fix init

* fix

* special treatment for sepconv1d

* style

* 🙏🏽

* add doc and cleanup


* fix doc

* fix doc again

* fix doc again

* Apply suggestions from code review

* make style

* Proposal that should work

* Remove needless code

* Fix test

* Apply suggestions from code review

* remove xnli_compute_metrics, add load_dataset, load_metric, set_seed,metric.compute,load_metric

* amend README

* removed data_args.task_name and replaced with task_name = "xnli"; use split function to load train and validation dataset separately; remove __post_init__; remove flag --task_name from README.

* removed dict task_to_keys, use str "xnli" instead of variable task_name, change preprocess_function to use examples["premise"], examples["hypothesis"] directly, remove sentence1_key and sentence2_key, change compute_metrics function to cater only to accuracy metric, add condition for train_langauge is None when using dataset.load_dataset()

* removed `torch.distributed.barrier()` and `import torch` as `from_pretrained` is able to do the work; amend README

examples/text-classification/README.md

@@ -143,23 +143,15 @@ Based on the script [`run_xnli.py`](https://github.com/huggingface/transformers/
 
 #### Fine-tuning on XNLI
 
-This example code fine-tunes mBERT (multi-lingual BERT) on the XNLI dataset. It runs in 106 mins
-on a single tesla V100 16GB. The data for XNLI can be downloaded with the following links and should be both saved (and un-zipped) in a
-`$XNLI_DIR` directory.
-
-* [XNLI 1.0](https://cims.nyu.edu/~sbowman/xnli/XNLI-1.0.zip)
-* [XNLI-MT 1.0](https://dl.fbaipublicfiles.com/XNLI/XNLI-MT-1.0.zip)
+This example code fine-tunes mBERT (multi-lingual BERT) on the XNLI dataset. It runs in 106 mins on a single tesla V100 16GB.
 
 ```bash
-export XNLI_DIR=/path/to/XNLI
-
 python run_xnli.py \
   --model_name_or_path bert-base-multilingual-cased \
   --language de \
   --train_language en \
   --do_train \
   --do_eval \
-  --data_dir $XNLI_DIR \
   --per_device_train_batch_size 32 \
   --learning_rate 5e-5 \
   --num_train_epochs 2.0 \