Adding TF translation example (#12667)

* Adding TF translation example

* Fixes and style pass for TF translation example

* Remove unused postprocess_text copied from run_summarization

* Adding README

* Review fixes

* Move changes to model.config to after we've initialized the model

examples/tensorflow/translation/README.md

+<!---
+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.
+-->
+
+# Translation example
+
+This script shows an example of training a *translation* 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 commands and caveats
+
+MBart and some T5 models require special handling.
+
+T5 models `t5-small`, `t5-base`, `t5-large`, `t5-3b` and `t5-11b` must use an additional argument: `--source_prefix "translate {source_lang} to {target_lang}"`. For example:
+
+```bash
+python run_translation.py \
+    --model_name_or_path t5-small \
+    --do_train \
+    --do_eval \
+    --source_lang en \
+    --target_lang ro \
+    --source_prefix "translate English to Romanian: " \
+    --dataset_name wmt16 \
+    --dataset_config_name ro-en \
+    --output_dir /tmp/tst-translation \
+    --per_device_train_batch_size=16 \
+    --per_device_eval_batch_size=16 \
+    --overwrite_output_dir
+```
+
+If you get a terrible BLEU score, make sure that you didn't forget to use the `--source_prefix` argument.
+
+For the aforementioned group of T5 models it's important to remember that if you switch to a different language pair, make sure to adjust the source and target values in all 3 language-specific command line argument: `--source_lang`, `--target_lang` and `--source_prefix`.
+
+MBart models require a different format for `--source_lang` and `--target_lang` values, e.g. instead of `en` it expects `en_XX`, for `ro` it expects `ro_RO`. The full MBart specification for language codes can be found [here](https://huggingface.co/facebook/mbart-large-cc25). For example:
+
+```bash
+python run_translation.py \
+    --model_name_or_path facebook/mbart-large-en-ro  \
+    --do_train \
+    --do_eval \
+    --dataset_name wmt16 \
+    --dataset_config_name ro-en \
+    --source_lang en_XX \
+    --target_lang ro_RO \
+    --output_dir /tmp/tst-translation \
+    --per_device_train_batch_size=16 \
+    --per_device_eval_batch_size=16 \
+    --overwrite_output_dir
+ ```