Add Flax BART pretraining script (#18297)

* add bart pretraining flax script

* fixup

* add bart pretraining flax script

* add BART to README

* add BART to README

* add BART to README

* add BART to README

* add BART to README

* add bos eos document

* Update README.md

* Update README.md

* Update examples/flax/language-modeling/run_bart_dlm_flax.py

Co-authored-by: Sanchit Gandhi <93869735+sanchit-gandhi@users.noreply.github.com>

* final

* final

* final

* remove use_auth_token ing from_config

Co-authored-by: Sanchit Gandhi <93869735+sanchit-gandhi@users.noreply.github.com>

examples/flax/language-modeling/README.md

@@ -338,6 +338,98 @@ of 2.36 and 57.0 respectively after 3 epochs on a single TPUv3-8.
 This should take around 4.5 hours.
 Training statistics can be accessed on directly on the 🤗 [hub](https://huggingface.co/patrickvonplaten/t5-base-norwegian/tensorboard)
 
+## BART: Denoising language modeling
+
+In the following, we demonstrate how to train a BART model 
+using denoising language modeling objective as introduced in [BART: Denoising Sequence-to-Sequence Pre-training for Natural Language Generation, Translation, and Comprehension](https://arxiv.org/abs/1910.13461).
+More specifically, we demonstrate how JAX/Flax can be leveraged 
+to pre-train [**`bart-base`**](https://huggingface.co/facebook/bart-base)
+in Norwegian on a single TPUv3-8 pod.
+
+The example script uses the 🤗 Datasets library. You can easily customize them to your needs if you need extra processing on your datasets.
+
+To setup all relevant files for training, let's create a directory.
+
+```bash
+mkdir ./norwegian-roberta-base
+```
+
+### Train tokenizer
+In the first step, we train a tokenizer to efficiently process the text input for the model. Similar to how it is shown in [How to train a new language model from scratch using Transformers and Tokenizers](https://huggingface.co/blog/how-to-train), we use a **`ByteLevelBPETokenizer`**.
+The tokenizer is trained on the complete Norwegian dataset of OSCAR
+and consequently saved in the cloned model directory.
+This can take up to 10 minutes depending on your hardware ☕.
+
+```python
+from datasets import load_dataset
+from tokenizers import trainers, Tokenizer, normalizers, ByteLevelBPETokenizer
+
+# load dataset
+dataset = load_dataset("oscar", "unshuffled_deduplicated_no", split="train")
+
+# Instantiate tokenizer
+tokenizer = ByteLevelBPETokenizer()
+
+def batch_iterator(batch_size=1000):
+    for i in range(0, len(dataset), batch_size):
+        yield dataset[i: i + batch_size]["text"]
+
+# Customized training
+tokenizer.train_from_iterator(batch_iterator(), vocab_size=50265, min_frequency=2, special_tokens=[
+    "<s>",
+    "<pad>",
+    "</s>",
+    "<unk>",
+    "<mask>",
+])
+
+# Save files to disk
+tokenizer.save("./norwegian-bart-base/tokenizer.json")
+```
+
+### Create configuration
+
+Next, we create the model's configuration file. This is as simple 
+as loading and storing [`**facebook/bart-base**`](https://huggingface.co/facebook/bart-base)
+in the local model folder:
+
+```python
+from transformers import BartConfig
+config = BartConfig.from_pretrained("facebook/bart-base", vocab_size=50265)
+config.save_pretrained("./norwegian-bart-base")
+```
+
+Great, we have set up our model repository. During training, we will automatically
+push the training logs and model weights to the repo.
+
+### Train model
+
+Next we can run the example script to pretrain the model:
+
+```bash
+python run_bart_dlm_flax.py \
+    --output_dir="./norwegian-bart-base" \
+    --config_name="./norwegian-bart-base" \
+    --tokenizer_name="./norwegian-bart-base" \
+    --dataset_name="oscar" \
+    --dataset_config_name="unshuffled_deduplicated_no" \
+    --max_seq_length="1024" \
+    --per_device_train_batch_size="32" \
+    --per_device_eval_batch_size="32" \
+    --learning_rate="1e-4" \
+    --warmup_steps="2000" \
+    --overwrite_output_dir \
+    --logging_steps="500" \
+    --save_steps="2000" \
+    --eval_steps="2000" \
+    --push_to_hub
+```
+
+Training should converge at a loss and accuracy 
+of 1.36 and 0.77 respectively after 3 epochs on a single TPUv3-8.
+This should take less than 6 hours.
+Training statistics can be accessed on [tfhub.dev](https://tensorboard.dev/experiment/Maw62QlaSXWS0MOf2V2lbg/).
+
 ## Runtime evaluation
 
 We also ran masked language modeling using PyTorch/XLA on a TPUv3-8, and PyTorch on 8 V100 GPUs. We report the