Add `examples/run_ner_no_trainer.py` (#10902)

* Add NER example with accelerate library

* This commit contains the first (yet really unfinished)
version of a script for showing how to train HuggingFace model
with their new accelerate library.

* Fix metric calculation

* make style quality

* mv ner_no_trainer to token-classification dir

* Delete --debug flag from running script

* hf_datasets -> raw_datasets

* Make a few slight adjustments

* Add an informative comment + rewrite a help comment

* Change header

* Fix a few things

* Enforce to use fast tokenizers only

* DataCollatorWithPadding -> DataCollatorForTokenClassification

* Change bash script: python3 -> accelerate launch

* make style

* Add a few missing things (see below)

* Add a max-lenghth padding to predictions and labels to
enable accelerate gather functionality

* Add PyTorch no trainer example to the example README.md

* Remove --do-train from args as being redundant for now

* DataCollatorWithPadding -> DataCollatorForTokenClassification

* Remove some obsolete args.do_train conditions from the script

* Delete --do_train from bash running script

* Delete use_slow_tokenizer from args

* Add unintentionally removed flag --label_all_tokens

* Delete --debug flag from running script

examples/token-classification/README.md

@@ -14,10 +14,12 @@ See the License for the specific language governing permissions and
 limitations under the License.
 -->
 
-## Token classification
+# Token classification
 
-Fine-tuning the library models for token classification task such as Named Entity Recognition (NER) or Parts-of-speech
-tagging (POS). The main scrip `run_ner.py` leverages the 🤗 Datasets library and the Trainer API. You can easily
+## PyTorch version
+
+Fine-tuning the library models for token classification task such as Named Entity Recognition (NER), Parts-of-speech
+tagging (POS) pr phrase extraction (CHUNKS). The main scrip `run_ner.py` leverages the 🤗 Datasets library and the Trainer API. You can easily
 customize it to your needs if you need extra processing on your datasets.
 
 It will either run on a datasets hosted on our [hub](https://huggingface.co/datasets) or with your own text files for
@@ -57,6 +59,74 @@ of the script.
 
 You can find the old version of the PyTorch script [here](https://github.com/huggingface/transformers/blob/master/examples/legacy/token-classification/run_ner.py).
 
+## Pytorch version, no Trainer
+
+Based on the script [run_ner_no_trainer.py](https://github.com/huggingface/transformers/blob/master/examples/token-classification/run_ner_no_trainer.py).
+
+Like `run_ner.py`, this script allows you to fine-tune any of the models on the [hub](https://huggingface.co/models) on a
+token classification task, either NER, POS or CHUNKS tasks or your own data in a csv or a JSON file. The main difference is that this
+script exposes the bare training loop, to allow you to quickly experiment and add any customization you would like.
+
+It offers less options than the script with `Trainer` (for instance you can easily change the options for the optimizer
+or the dataloaders directly in the script) but still run in a distributed setup, on TPU and supports mixed precision by
+the mean of the [🤗 `Accelerate`](https://github.com/huggingface/accelerate) library. You can use the script normally
+after installing it:
+
+```bash
+pip install accelerate
+```
+
+then
+
+```bash
+export TASK_NAME=ner
+
+python run_ner_no_trainer.py \
+  --model_name_or_path bert-base-cased \
+  --task_name $TASK_NAME \
+  --max_seq_length 128 \
+  --per_device_train_batch_size 32 \
+  --learning_rate 2e-5 \
+  --num_train_epochs 3 \
+  --output_dir /tmp/$TASK_NAME/
+```
+
+You can then use your usual launchers to run in it in a distributed environment, but the easiest way is to run
+
+```bash
+accelerate config
+```
+
+and reply to the questions asked. Then
+
+```bash
+accelerate test
+```
+
+that will check everything is ready for training. Finally, you cna launch training with
+
+```bash
+export TASK_NAME=ner
+
+accelerate launch run_ner_no_trainer.py \
+  --model_name_or_path bert-base-cased \
+  --task_name $TASK_NAME \
+  --max_seq_length 128 \
+  --per_device_train_batch_size 32 \
+  --learning_rate 2e-5 \
+  --num_train_epochs 3 \
+  --output_dir /tmp/$TASK_NAME/
+```
+
+This command is the same and will work for:
+
+- a CPU-only setup
+- a setup with one GPU
+- a distributed training with several GPUs (single or multi node)
+- a training on TPUs
+
+Note that this library is in alpha release so your feedback is more than welcome if you encounter any problem using it.
+
 ### TensorFlow version
 
 The following examples are covered in this section:

examples/token-classification/run_no_trainer.sh

+# Copyright 2020 The HuggingFace Team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+accelerate launch run_ner_no_trainer.py \
+  --model_name_or_path bert-base-uncased \
+  --dataset_name conll2003 \
+  --output_dir /tmp/test-ner \
+  --pad_to_max_length \
+  --task_name ner \
+  --return_entity_level_metrics