[ASR Pipeline] Clarify return timestamps (#25344)

* [ASR Pipeline] Clarify return timestamps

* fix indentation

* fix ctc check

* fix ctc error message!

* fix test

* fix other test

* add new tests

* final comment

tests/pipelines/test_pipelines_automatic_speech_recognition.py

@@ -136,7 +136,7 @@ class AutomaticSpeechRecognitionPipelineTests(unittest.TestCase):
         else:
             # Non CTC models cannot use return_timestamps
             with self.assertRaisesRegex(
-                ValueError, "^We cannot return_timestamps yet on non-ctc models apart from Whisper !$"
+                ValueError, "^We cannot return_timestamps yet on non-CTC models apart from Whisper!$"
             ):
                 outputs = speech_recognizer(audio, return_timestamps="char")
 
@@ -161,7 +161,7 @@ class AutomaticSpeechRecognitionPipelineTests(unittest.TestCase):
 
         # Non CTC models cannot use return_timestamps
         with self.assertRaisesRegex(
-            ValueError, "^We cannot return_timestamps yet on non-ctc models apart from Whisper !$"
+            ValueError, "^We cannot return_timestamps yet on non-CTC models apart from Whisper!$"
         ):
             _ = speech_recognizer(waveform, return_timestamps="char")
 
@@ -261,6 +261,11 @@ class AutomaticSpeechRecognitionPipelineTests(unittest.TestCase):
                 ],
             },
         )
+        # CTC + LM models cannot use return_timestamps="char"
+        with self.assertRaisesRegex(
+            ValueError, "^CTC with LM can only predict word level timestamps, set `return_timestamps='word'`$"
+        ):
+            _ = speech_recognizer(filename, return_timestamps="char")
 
     @require_tf
     def test_small_model_tf(self):
@@ -750,6 +755,14 @@ class AutomaticSpeechRecognitionPipelineTests(unittest.TestCase):
         )
         # fmt: on
 
+        # Whisper can only predict segment level timestamps or word level, not character level
+        with self.assertRaisesRegex(
+            ValueError,
+            "^Whisper cannot return `char` timestamps, only word level or segment level timestamps. "
+            "Use `return_timestamps='word'` or `return_timestamps=True` respectively.$",
+        ):
+            _ = speech_recognizer(filename, return_timestamps="char")
+
     @slow
     @require_torch
     @require_torchaudio
@@ -1082,6 +1095,13 @@ class AutomaticSpeechRecognitionPipelineTests(unittest.TestCase):
                 ],
             },
         )
+        # CTC models must specify return_timestamps type - cannot set `return_timestamps=True` blindly
+        with self.assertRaisesRegex(
+            ValueError,
+            "^CTC can either predict character (char) level timestamps, or word level timestamps."
+            "Set `return_timestamps='char'` or `return_timestamps='word'` as required.$",
+        ):
+            _ = speech_recognizer(audio, return_timestamps=True)
 
     @require_torch
     @slow