[examples] unit test for run_bart_sum (#3544)

- adds pytorch-lightning dependency

examples/summarization/bart/run_train_tiny.sh

@@ -0,0 +1,33 @@
+# Script for verifying that run_bart_sum can be invoked from its directory
+
+# Get tiny dataset with cnn_dm format (4 examples for train, val, test)
+wget https://s3.amazonaws.com/datasets.huggingface.co/summarization/cnn_tiny.tgz
+tar -xzvf cnn_tiny.tgz
+rm cnn_tiny.tgz
+
+export OUTPUT_DIR_NAME=bart_utest_output
+export CURRENT_DIR=${PWD}
+export OUTPUT_DIR=${CURRENT_DIR}/${OUTPUT_DIR_NAME}
+
+# Make output directory if it doesn't exist
+mkdir -p $OUTPUT_DIR
+
+# Add parent directory to python path to access transformer_base.py and utils.py
+export PYTHONPATH="../../":"${PYTHONPATH}"
+python run_bart_sum.py \
+--data_dir=cnn_tiny/ \
+--model_type=bart \
+--model_name_or_path=sshleifer/bart-tiny-random \
+--learning_rate=3e-5 \
+--train_batch_size=2 \
+--eval_batch_size=2 \
+--output_dir=$OUTPUT_DIR \
+--num_train_epochs=1  \
+--n_gpu=0 \
+--do_train $@
+
+rm -rf cnn_tiny
+rm -rf $OUTPUT_DIR
+
+
+