[test] replace capsys with the more refined CaptureStderr/CaptureStdout (#6422)

* replace capsys with the more refined CaptureStderr/CaptureStdout

* Update examples/seq2seq/test_seq2seq_examples.py
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

examples/seq2seq/test_seq2seq_examples.py

@@ -15,7 +15,7 @@ from torch.utils.data import DataLoader
 
 import lightning_base
 from transformers import AutoModelForSeq2SeqLM, AutoTokenizer
-from transformers.testing_utils import require_multigpu
+from transformers.testing_utils import CaptureStderr, CaptureStdout, require_multigpu
 
 from .distillation import distill_main, evaluate_checkpoint
 from .finetune import SummarizationModule, main
@@ -329,7 +329,7 @@ def test_finetune_extra_model_args():
     assert str(excinfo.value) == f"model config doesn't have a `{unsupported_param}` attribute"
 
 
-def test_finetune_lr_shedulers(capsys):
+def test_finetune_lr_schedulers():
     args_d: dict = CHEAP_ARGS.copy()
 
     task = "summarization"
@@ -361,23 +361,23 @@ def test_finetune_lr_shedulers(capsys):
 
     # --help test
     with pytest.raises(SystemExit) as excinfo:
+        with CaptureStdout() as cs:
             args = parser.parse_args(args)
         assert False, "--help is expected to sys.exit"
     assert excinfo.type == SystemExit
-    captured = capsys.readouterr()
     expected = lightning_base.arg_to_scheduler_metavar
-    assert expected in captured.out, "--help is expected to list the supported schedulers"
+    assert expected in cs.out, "--help is expected to list the supported schedulers"
 
     # --lr_scheduler=non_existing_scheduler test
     unsupported_param = "non_existing_scheduler"
     args = {f"--lr_scheduler={unsupported_param}"}
     with pytest.raises(SystemExit) as excinfo:
+        with CaptureStderr() as cs:
             args = parser.parse_args(args)
         assert False, "invalid argument is expected to sys.exit"
     assert excinfo.type == SystemExit
-    captured = capsys.readouterr()
     expected = f"invalid choice: '{unsupported_param}'"
-    assert expected in captured.err, f"should have bailed on invalid choice of scheduler {unsupported_param}"
+    assert expected in cs.err, f"should have bailed on invalid choice of scheduler {unsupported_param}"
 
     # --lr_scheduler=existing_scheduler test
     supported_param = "cosine"