Switch from using sum for flattening lists of lists in group_texts (#14472)

* remove sum for list flattening

* change to chain(*)

* make chain object a list

* delete empty lines

per sgugger's suggestions

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

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

examples/pytorch/language-modeling/run_plm.py

@@ -23,6 +23,7 @@ import math
 import os
 import sys
 from dataclasses import dataclass, field
+from itertools import chain
 from typing import Optional
 
 import datasets
@@ -403,7 +404,7 @@ def main():
         # max_seq_length.
         def group_texts(examples):
             # Concatenate all texts.
-            concatenated_examples = {k: sum(examples[k], []) for k in examples.keys()}
+            concatenated_examples = {k: list(chain(*examples[k])) for k in examples.keys()}
             total_length = len(concatenated_examples[list(examples.keys())[0]])
             # We drop the small remainder, we could add padding if the model supported it instead of this drop, you can
             # customize this part to your needs.