[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

src/transformers/pipelines/automatic_speech_recognition.py

@@ -156,8 +156,7 @@ class AutomaticSpeechRecognitionPipeline(ChunkPipeline):
         feature_extractor ([`SequenceFeatureExtractor`]):
             The feature extractor that will be used by the pipeline to encode waveform for the model.
         chunk_length_s (`float`, *optional*, defaults to 0):
-            The input length for in each chunk. If `chunk_length_s = 0` then chunking is disabled (default). Only
-            available for CTC models, e.g. [`Wav2Vec2ForCTC`].
+            The input length for in each chunk. If `chunk_length_s = 0` then chunking is disabled (default).
 
             <Tip>
 
@@ -247,14 +246,29 @@ class AutomaticSpeechRecognitionPipeline(ChunkPipeline):
                       np.array}` with optionally a `"stride": (left: int, right: int)` than can ask the pipeline to
                       treat the first `left` samples and last `right` samples to be ignored in decoding (but used at
                       inference to provide more context to the model). Only use `stride` with CTC models.
-            return_timestamps (*optional*, `str`):
-                Only available for pure CTC models. If set to `"char"`, the pipeline will return timestamps along the
-                text for every character in the text. For instance if you get `[{"text": "h", "timestamp": (0.5, 0.6)},
-                {"text": "i", "timestamp": (0.7, 0.9)}]`, then it means the model predicts that the letter "h" was
-                pronounced after `0.5` and before `0.6` seconds. If set to `"word"`, the pipeline will return
-                timestamps along the text for every word in the text. For instance if you get `[{"text": "hi ",
-                "timestamp": (0.5, 0.9)}, {"text": "there", "timestamp": (1.0, 1.5)}]`, then it means the model
-                predicts that the word "hi" was pronounced after `0.5` and before `0.9` seconds.
+            return_timestamps (*optional*, `str` or `bool`):
+                Only available for pure CTC models (Wav2Vec2, HuBERT, etc) and the Whisper model. Not available for
+                other sequence-to-sequence models.
+
+                For CTC models, timestamps can take one of two formats:
+                    - `"char"`: the pipeline will return timestamps along the text for every character in the text. For
+                        instance, if you get `[{"text": "h", "timestamp": (0.5, 0.6)}, {"text": "i", "timestamp": (0.7,
+                        0.9)}]`, then it means the model predicts that the letter "h" was spoken after `0.5` and before
+                        `0.6` seconds.
+                    - `"word"`: the pipeline will return timestamps along the text for every word in the text. For
+                        instance, if you get `[{"text": "hi ", "timestamp": (0.5, 0.9)}, {"text": "there", "timestamp":
+                        (1.0, 1.5)}]`, then it means the model predicts that the word "hi" was spoken after `0.5` and
+                        before `0.9` seconds.
+
+                For the Whisper model, timestamps can take one of two formats:
+                    - `"word"`: same as above for word-level CTC timestamps. Word-level timestamps are predicted
+                        through the *dynamic-time warping (DTW)* algorithm, an approximation to word-level timestamps
+                        by inspecting the cross-attention weights.
+                    - `True`: the pipeline will return timestamps along the text for *segments* of words in the text.
+                        For instance, if you get `[{"text": " Hi there!", "timestamp": (0.5, 1.5)}]`, then it means the
+                        model predicts that the segment "Hi there!" was spoken after `0.5` and before `1.5` seconds.
+                        Note that a segment of text refers to a sequence of one or more words, rather than individual
+                        words as with word-level timestamps.
             generate_kwargs (`dict`, *optional*):
                 The dictionary of ad-hoc parametrization of `generate_config` to be used for the generation call. For a
                 complete overview of generate, check the [following
@@ -264,7 +278,7 @@ class AutomaticSpeechRecognitionPipeline(ChunkPipeline):
 
         Return:
             `Dict`: A dictionary with the following keys:
-                - **text** (`str` ) -- The recognized text.
+                - **text** (`str`): The recognized text.
                 - **chunks** (*optional(, `List[Dict]`)
                     When using `return_timestamps`, the `chunks` will become a list containing all the various text
                     chunks identified by the model, *e.g.* `[{"text": "hi ", "timestamp": (0.5, 0.9)}, {"text":
@@ -308,6 +322,20 @@ class AutomaticSpeechRecognitionPipeline(ChunkPipeline):
         if decoder_kwargs is not None:
             postprocess_params["decoder_kwargs"] = decoder_kwargs
         if return_timestamps is not None:
+            if self.type == "seq2seq" and return_timestamps:
+                raise ValueError("We cannot return_timestamps yet on non-CTC models apart from Whisper!")
+            if self.type == "ctc_with_lm" and return_timestamps != "word":
+                raise ValueError("CTC with LM can only predict word level timestamps, set `return_timestamps='word'`")
+            if self.type == "ctc" and return_timestamps not in ["char", "word"]:
+                raise ValueError(
+                    "CTC can either predict character (char) level timestamps, or word level timestamps."
+                    "Set `return_timestamps='char'` or `return_timestamps='word'` as required."
+                )
+            if self.type == "seq2seq_whisper" and return_timestamps == "char":
+                raise ValueError(
+                    "Whisper cannot return `char` timestamps, only word level or segment level timestamps. "
+                    "Use `return_timestamps='word'` or `return_timestamps=True` respectively."
+                )
             forward_params["return_timestamps"] = return_timestamps
             postprocess_params["return_timestamps"] = return_timestamps
         if return_language is not None:
@@ -497,13 +525,6 @@ class AutomaticSpeechRecognitionPipeline(ChunkPipeline):
         # Optional return types
         optional = {}
 
-        if return_timestamps and self.type == "seq2seq":
-            raise ValueError("We cannot return_timestamps yet on non-ctc models apart from Whisper !")
-        if return_timestamps == "char" and self.type == "ctc_with_lm":
-            raise ValueError("CTC with LM cannot return `char` timestamps, only `word`")
-        if return_timestamps == "char" and self.type == "seq2seq_whisper":
-            raise ValueError("Whisper cannot return `char` timestamps, use `True` or `word` instead.")
-
         if return_language is not None and self.type != "seq2seq_whisper":
             raise ValueError("Only whisper can return language for now.")
 

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