[Examples] make default preprocessing_num_workers=1 (#19684)

* [Examples] make default preprocessing_num_workers=1

* [Examples] revert changes in research projects

examples/pytorch/question-answering/run_qa_beam_search_no_trainer.py

@@ -104,7 +104,7 @@ def parse_args():
         "--train_file", type=str, default=None, help="A csv or a json file containing the training data."
     )
     parser.add_argument(
-        "--preprocessing_num_workers", type=int, default=4, help="A csv or a json file containing the training data."
+        "--preprocessing_num_workers", type=int, default=1, help="A csv or a json file containing the training data."
     )
     parser.add_argument("--do_predict", action="store_true", help="Eval the question answering model")
     parser.add_argument(

examples/pytorch/question-answering/run_qa_no_trainer.py

@@ -108,7 +108,7 @@ def parse_args():
         "--train_file", type=str, default=None, help="A csv or a json file containing the training data."
     )
     parser.add_argument(
-        "--preprocessing_num_workers", type=int, default=4, help="A csv or a json file containing the training data."
+        "--preprocessing_num_workers", type=int, default=1, help="A csv or a json file containing the training data."
     )
     parser.add_argument("--do_predict", action="store_true", help="To do prediction on the question answering model")
     parser.add_argument(