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/flax/language-modeling/run_mlm_flax.py

@@ -25,6 +25,7 @@ import os
 import sys
 import time
 from dataclasses import dataclass, field
+from itertools import chain
 
 # You can also adapt this script on your own masked language modeling task. Pointers for this are left as comments.
 from pathlib import Path
@@ -453,7 +454,7 @@ if __name__ == "__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.