TF summarization example (#12617)

* Adding a TF summarization example

* Style pass

* Style fixes

* Updates for review comments

* Adding README

* Style pass

* Remove unused import

examples/tensorflow/summarization/README.md

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+<!---
+Copyright 2021 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.
+-->
+
+# Summarization example
+
+This script shows an example of training a *summarization* model with the 🤗 Transformers library.
+For straightforward use-cases you may be able to use these scripts without modification, although we have also
+included comments in the code to indicate areas that you may need to adapt to your own projects.
+
+### Multi-GPU and TPU usage
+
+By default, these scripts use a `MirroredStrategy` and will use multiple GPUs effectively if they are available. TPUs
+can also be used by passing the name of the TPU resource with the `--tpu` argument.
+
+### Example command
+```
+python run_summarization.py  \
+--model_name_or_path facebook/bart-base \
+--dataset_name cnn_dailymail \
+--dataset_config "3.0.0" \
+--output_dir /tmp/tst-summarization  \
+--per_device_train_batch_size 8 \
+--per_device_eval_batch_size 16 \
+--num_train_epochs 3 \
+--do_train \
+--do_eval
+```