[s2s] fix nltk pytest race condition with FileLock (#7515)

examples/seq2seq/sentence_splitter.py

 import re
 
+from filelock import FileLock
+
 
 try:
     import nltk
@@ -9,13 +11,12 @@ except (ImportError, ModuleNotFoundError):
     NLTK_AVAILABLE = False
 
 if NLTK_AVAILABLE:
-    try:
+    with FileLock("a_random_string") as lock:
         nltk.download("punkt", quiet=True)
-    except FileExistsError:  # multiprocessing race condition
-        pass
 
 
 def add_newline_to_end_of_each_sentence(x: str) -> str:
+    """This was added to get rougeLsum scores matching published rougeL scores for BART and PEGASUS."""
     re.sub("<n>", "", x)  # remove pegasus newline char
-    assert NLTK_AVAILABLE, "nltk must be installed to separate newlines betwee sentences. (pip install nltk)"
+    assert NLTK_AVAILABLE, "nltk must be installed to separate newlines between sentences. (pip install nltk)"
     return "\n".join(nltk.sent_tokenize(x))