[s2s] rougeLSum expects \n between sentences (#7410)

Co-authored-by: Swetha Mandava <smandava@nvidia.com>

examples/seq2seq/sentence_splitter.py

@@ -0,0 +1,21 @@
+import re
+
+
+try:
+    import nltk
+
+    NLTK_AVAILABLE = True
+except (ImportError, ModuleNotFoundError):
+    NLTK_AVAILABLE = False
+
+if NLTK_AVAILABLE:
+    try:
+        nltk.download("punkt", quiet=True)
+    except FileExistsError:  # multiprocessing race condition
+        pass
+
+
+def add_newline_to_end_of_each_sentence(x: str) -> str:
+    re.sub("<n>", "", x)  # remove pegasus newline char
+    assert NLTK_AVAILABLE, "nltk must be installed to separate newlines betwee sentences. (pip install nltk)"
+    return "\n".join(nltk.sent_tokenize(x))