Misc. fixes for Pytorch QA examples: (#16958)

1. Fixes evaluation errors popping up when you train/eval on squad v2 (one was newly encountered and one that was previously reported Running SQuAD 1.0 sample command raises IndexError #15401 but not completely fixed).
2. Removes boolean arguments that don't use store_true. Please, don't use these: *ANY non-empty string is being converted to True in this case and this clearly is not the desired behavior (and it creates a LOT of confusion).
3. All no-trainer test scripts are now saving metric values in the same way (with the right prefix eval_), which is consistent with the trainer-based versions.
4. Adds forgotten model.eval() in the no-trainer versions. This improved some results, but not everything (see the discussion in the end). Please, see the F1 scores and the discussion below.

examples/pytorch/test_accelerate_examples.py

@@ -200,7 +200,7 @@ class ExamplesTestsNoTrainer(TestCasePlus):
         testargs = f"""
             run_qa_no_trainer.py
             --model_name_or_path bert-base-uncased
-            --version_2_with_negative=False
+            --version_2_with_negative
             --train_file tests/fixtures/tests_samples/SQUAD/sample.json
             --validation_file tests/fixtures/tests_samples/SQUAD/sample.json
             --output_dir {tmp_dir}
@@ -216,6 +216,7 @@ class ExamplesTestsNoTrainer(TestCasePlus):
         with patch.object(sys, "argv", testargs):
             run_squad_no_trainer.main()
             result = get_results(tmp_dir)
+            # Because we use --version_2_with_negative the testing script uses SQuAD v2 metrics.
             self.assertGreaterEqual(result["eval_f1"], 30)
             self.assertGreaterEqual(result["eval_exact"], 30)
             self.assertTrue(os.path.exists(os.path.join(tmp_dir, "epoch_0")))