add readme for flax clm (#12111)

* add readme for flax clm

* use section link for tokenizer

* Apply suggestions from code review
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* update metrics
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

examples/flax/language-modeling/README.md

@@ -125,6 +125,68 @@ Training statistics can be accessed on [tfhub.de](https://tensorboard.dev/experi
 For a step-by-step walkthrough of how to do masked language modeling in Flax, please have a 
 look at [this](https://colab.research.google.com/github/huggingface/notebooks/blob/master/examples/masked_language_modeling_flax.ipynb) google colab.
 
+## Causal language modeling
+
+In the following, we demonstrate how to train an auto-regressive causal transformer model 
+in JAX/Flax.
+More specifically, we pretrain a randomely initialized [**`gpt2`**](https://huggingface.co/gpt2) model in Norwegian on a single TPUv3-8.
+to pre-train 124M [**`gpt2`**](https://huggingface.co/gpt2)
+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.
+
+Let's start by creating a folder to save the trained model and a symbolic link to the `run_clm_flax.py` script.
+
+```bash
+export MODEL_DIR="./norwegian-gpt2"
+mkdir -p ${MODEL_DIR}
+ln -s ~/transformers/examples/flax/language-modeling/run_clm_flax.py run_clm_flax.py
+```
+
+Next, we'll follow the same steps as above in [Train tokenizer](#train-tokenizer) to train the tokenizer.
+
+### Create configuration
+
+Next, we create the model's configuration file. This is as simple 
+as loading and storing [`**gpt2**`](https://huggingface.co/gpt2)
+in the local model folder:
+
+```python
+from transformers import GPT2Config
+
+model_dir = "./norwegian-gpt2"  # ${MODEL_DIR}
+
+config = GPT2Config.from_pretrained("gpt2", resid_pdrop=0.0, embd_pdrop=0.0, attn_pdrop=0.0)
+config.save_pretrained(model_dir)
+```
+
+### Train model
+
+Next we can run the example script to pretrain the model:
+
+```bash
+./run_clm_flax.py \
+    --output_dir="./runs" \
+    --model_type="gpt2" \
+    --config_name="${MODEL_DIR}" \
+    --tokenizer_name="${MODEL_DIR}" \
+    --dataset_name="oscar" \
+    --dataset_config_name="unshuffled_deduplicated_no" \
+    --do_train --do_eval \
+    --block_size="512" \
+    --per_device_train_batch_size="64" \
+    --per_device_eval_batch_size="64" \
+    --learning_rate="5e-3" --warmup_steps="1000" \
+    --adam_beta1="0.9" --adam_beta2="0.98" --weight_decay="0.01" \
+    --overwrite_output_dir \
+    --num_train_epochs="20" \
+```
+
+Training should converge at a loss and perplexity 
+of 3.24 and 25.72 respectively after 20 epochs on a single TPUv3-8.
+This should take less than ~21 hours.
+Training statistics can be accessed on [tfhub.de](https://tensorboard.dev/experiment/2zEhLwJ0Qp2FAkI3WVH9qA).
+
 
 ## Runtime evaluation