[WIP][examples/seq2seq] move old s2s scripts to legacy (#10136)

* move old s2s scripts to legacy

* add the tests back

* proper rename

* restore

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

Co-authored-by: Stas Bekman <stas@stason.org>
Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

examples/legacy/seq2seq/sentence_splitter.py

@@ -0,0 +1,35 @@
+# Copyright 2020 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.
+import re
+
+from filelock import FileLock
+
+
+try:
+    import nltk
+
+    NLTK_AVAILABLE = True
+except (ImportError, ModuleNotFoundError):
+    NLTK_AVAILABLE = False
+
+if NLTK_AVAILABLE:
+    with FileLock(".lock") as lock:
+        nltk.download("punkt", quiet=True)
+
+
+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 between sentences. (pip install nltk)"
+    return "\n".join(nltk.sent_tokenize(x))