[examples] consolidate summarization examples (#4837)

examples/summarization/README.md

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+### Get CNN Data
+Both types of models do require CNN data and follow different procedures of obtaining so.
+
+#### For BART models
+To be able to reproduce the authors' results on the CNN/Daily Mail dataset you first need to download both CNN and Daily Mail datasets [from Kyunghyun Cho's website](https://cs.nyu.edu/~kcho/DMQA/) (the links next to "Stories") in the same folder. Then uncompress the archives by running:
+
+```bash
+wget https://s3.amazonaws.com/datasets.huggingface.co/summarization/cnn_dm.tgz
+tar -xzvf cnn_dm.tgz
+```
+
+this should make a directory called cnn_dm/ with files like `test.source`.
+To use your own data, copy that files format. Each article to be summarized is on its own line.
+
+#### For T5 models
+First, you need to download the CNN data. It's about ~400 MB and can be downloaded by
+running
+
+```bash
+python download_cnn_daily_mail.py cnn_articles_input_data.txt cnn_articles_reference_summaries.txt
+```
+
+You should confirm that each file has 11490 lines:
+
+```bash
+wc -l cnn_articles_input_data.txt # should print 11490
+wc -l cnn_articles_reference_summaries.txt # should print 11490
+```
+
+### Evaluation
+
+To create summaries for each article in dataset, run:
+```bash
+python evaluate_cnn.py <path_to_test.source> test_generations.txt <model-name>
+```
+The default batch size, 8, fits in 16GB GPU memory, but may need to be adjusted to fit your system.
+
+### Training
+Run/modify `finetune_bart.sh` or `finetune_t5.sh`
+
+## (WIP) Rouge Scores
+
+To create summaries for each article in dataset and also calculate rouge scores run:
+```bash
+python evaluate_cnn.py <path_to_test.source> test_generations.txt <model-name> --reference_path <path_to_correct_summaries> --score_path <path_to_save_rouge_scores>
+```
+The rouge scores "rouge1, rouge2, rougeL" are automatically created and saved in ``<path_to_save_rouge_scores>``.
+
+### Stanford CoreNLP Setup
+```
+ptb_tokenize () {
+    cat $1 | java edu.stanford.nlp.process.PTBTokenizer -ioFileList -preserveLines > $2
+}
+
+sudo apt install openjdk-8-jre-headless
+sudo apt-get install ant
+wget http://nlp.stanford.edu/software/stanford-corenlp-full-2018-10-05.zip
+unzip stanford-corenlp-full-2018-10-05.zip
+cd stanford-corenlp-full-2018-10-05
+export CLASSPATH=stanford-corenlp-3.9.2.jar:stanford-corenlp-3.9.2-models.jar
+```
+Then run `ptb_tokenize` on `test.target` and your generated hypotheses.
+### Rouge Setup
+Install `files2rouge` following the instructions at [here](https://github.com/pltrdy/files2rouge).
+I also needed to run `sudo apt-get install libxml-parser-perl`
+
+```python
+from files2rouge import files2rouge
+from files2rouge import settings
+files2rouge.run(<path_to_tokenized_hypo>,
+                <path_to_tokenized_target>,
+               saveto='rouge_output.txt')
+```