Repository: huggingface/transformers (Python)
Task: Bug Fix

Issue:
wav2vec processor batching logic is too restrictive
### System Info

transformers version at the time of writing is `4.26.1`


### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```python
# !pip install transformers torch # in jupyter notebook
from transformers import Wav2Vec2Processor
import torch
import numpy as np

batch = 4

# create Wav2Vec2Processor
processor = Wav2Vec2Processor.from_pretrained("facebook/hubert-large-ls960-ft")
# generate random input tensor
input_tensor = torch.tensor(np.random.randn(batch, 10, 10))
# pass input tensor through processor
output = processor(input_tensor, return_tensors="pt")
print(output["input_values"].shape) # 1 x 4 x 10 x 10
```


### Expected behavior

It seems reasonable that an input could be of shape `batch x d_1 x d_2 ...` and I'd expect the output to have the same shape. However, [here](https://github.com/huggingface/transformers/blob/main/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py#L184) the code has an extra check for type list or tuple that results in it misinterpreting the input as a single example.

Side note: I'm unsure what to infer from the type checking logic because it doesn't match the type hints i.e. `tuple` isn't supposed to be possible here anyways, according to the `__call__` type hint. I did check some other examples of `is_batched` appearing in the `src/transformers/models` directory and they look similar but unexpected.


Discussion:
cc @sanchit-gandhi @ArthurZucker 
Hey @LWprogramming! Thanks for the comprehensive issue description - I agree that the logic for checking if the input `is_batched` is broken when the input is a batched numpy array, e.g. the feature extractor **should** set `is_batched=True` when the numpy array is 2-d, but currently does not:
https://github.com/huggingface/transformers/blob/57f25f4b7fb85ff069f8701372710b2a3207bf2d/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py#L184-L187

Would you like to open a PR to fix this? 🤗 We can just do one additional check to set `is_batched = True` if the input is a 2-d numpy array. Note that it should be 2-d with dims [batch, audio_input] and not 3-d since we only expect mono channel input to the feature extractor.
Hey @LWprogramming! Just checking-in to see whether you'd like to open a PR to fix the issue you uncovered? Think you're in a good position to submit a clean fix! 🤗
Hi! I'll take care of it, got preoccupied with some irl stuff that came up the past few weeks but things should be settling down soon :)
That's awesome @LWprogramming! Excited for the PR 🤗 Feel free to tag me as soon as it's ready and I'll get you a review

Patch:
diff --git a/src/transformers/feature_extraction_sequence_utils.py b/src/transformers/feature_extraction_sequence_utils.py
--- a/src/transformers/feature_extraction_sequence_utils.py
+++ b/src/transformers/feature_extraction_sequence_utils.py
@@ -140,7 +140,7 @@ def pad(
             return_attention_mask if return_attention_mask is not None else self.return_attention_mask
         )
 
-        if not required_input:
+        if len(required_input) == 0:
             if return_attention_mask:
                 processed_features["attention_mask"] = []
             return processed_features
diff --git a/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py b/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py
--- a/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py
+++ b/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py
@@ -117,7 +117,8 @@ def __call__(
         Args:
             raw_speech (`np.ndarray`, `List[float]`, `List[np.ndarray]`, `List[List[float]]`):
                 The sequence or batch of sequences to be padded. Each sequence can be a numpy array, a list of float
-                values, a list of numpy arrays or a list of list of float values.
+                values, a list of numpy arrays or a list of list of float values. Must be mono channel audio, not
+                stereo, i.e. single float per timestep.
             padding (`bool`, `str` or [`~utils.PaddingStrategy`], *optional*, defaults to `False`):
                 Select a strategy to pad the returned sequences (according to the model's padding side and padding
                 index) among:
@@ -181,9 +182,11 @@ def __call__(
                 "Failing to do so can result in silent errors that might be hard to debug."
             )
 
-        is_batched = bool(
-            isinstance(raw_speech, (list, tuple))
-            and (isinstance(raw_speech[0], np.ndarray) or isinstance(raw_speech[0], (tuple, list)))
+        is_batched_numpy = isinstance(raw_speech, np.ndarray) and len(raw_speech.shape) > 1
+        if is_batched_numpy and len(raw_speech.shape) > 2:
+            raise ValueError(f"Only mono-channel audio is supported for input to {self}")
+        is_batched = is_batched_numpy or (
+            isinstance(raw_speech, (list, tuple)) and (isinstance(raw_speech[0], (np.ndarray, tuple, list)))
         )
 
         # always return batch
diff --git a/src/transformers/models/wav2vec2/tokenization_wav2vec2.py b/src/transformers/models/wav2vec2/tokenization_wav2vec2.py
--- a/src/transformers/models/wav2vec2/tokenization_wav2vec2.py
+++ b/src/transformers/models/wav2vec2/tokenization_wav2vec2.py
@@ -817,12 +817,15 @@ def __call__(
         Args:
             raw_speech (`np.ndarray`, `List[float]`, `List[np.ndarray]`, `List[List[float]]`):
                 The sequence or batch of sequences to be padded. Each sequence can be a numpy array, a list of float
-                values, a list of numpy arrayr or a list of list of float values.
+                values, a list of numpy array or a list of list of float values. Must be mono channel audio, not
+                stereo, i.e. single float per timestep.
         """
 
-        is_batched = bool(
-            isinstance(raw_speech, (list, tuple))
-            and (isinstance(raw_speech[0], np.ndarray) or isinstance(raw_speech[0], (tuple, list)))
+        is_batched_numpy = isinstance(raw_speech, np.ndarray) and len(raw_speech.shape) > 1
+        if is_batched_numpy and len(raw_speech.shape) > 2:
+            raise ValueError(f"Only mono-channel audio is supported for input to {self}")
+        is_batched = is_batched_numpy or (
+            isinstance(raw_speech, (list, tuple)) and (isinstance(raw_speech[0], (np.ndarray, tuple, list)))
         )
 
         # make sure input is in list format


Test patch:
diff --git a/tests/models/wav2vec2/test_feature_extraction_wav2vec2.py b/tests/models/wav2vec2/test_feature_extraction_wav2vec2.py
--- a/tests/models/wav2vec2/test_feature_extraction_wav2vec2.py
+++ b/tests/models/wav2vec2/test_feature_extraction_wav2vec2.py
@@ -123,6 +123,14 @@ def test_call(self):
         for enc_seq_1, enc_seq_2 in zip(encoded_sequences_1, encoded_sequences_2):
             self.assertTrue(np.allclose(enc_seq_1, enc_seq_2, atol=1e-3))
 
+        # Test 2-D numpy arrays are batched.
+        speech_inputs = [floats_list((1, x))[0] for x in (800, 800, 800)]
+        np_speech_inputs = np.asarray(speech_inputs)
+        encoded_sequences_1 = feat_extract(speech_inputs, return_tensors="np").input_values
+        encoded_sequences_2 = feat_extract(np_speech_inputs, return_tensors="np").input_values
+        for enc_seq_1, enc_seq_2 in zip(encoded_sequences_1, encoded_sequences_2):
+            self.assertTrue(np.allclose(enc_seq_1, enc_seq_2, atol=1e-3))
+
     def test_zero_mean_unit_variance_normalization_np(self):
         feat_extract = self.feature_extraction_class(**self.feat_extract_tester.prepare_feat_extract_dict())
         speech_inputs = [floats_list((1, x))[0] for x in range(800, 1400, 200)]
diff --git a/tests/models/wav2vec2/test_tokenization_wav2vec2.py b/tests/models/wav2vec2/test_tokenization_wav2vec2.py
--- a/tests/models/wav2vec2/test_tokenization_wav2vec2.py
+++ b/tests/models/wav2vec2/test_tokenization_wav2vec2.py
@@ -164,6 +164,14 @@ def test_call(self):
         for enc_seq_1, enc_seq_2 in zip(encoded_sequences_1, encoded_sequences_2):
             self.assertTrue(np.allclose(enc_seq_1, enc_seq_2, atol=1e-3))
 
+        # Test 2-D numpy arrays are batched.
+        speech_inputs = [floats_list((1, x))[0] for x in (800, 800, 800)]
+        np_speech_inputs = np.asarray(speech_inputs)
+        encoded_sequences_1 = tokenizer(speech_inputs, return_tensors="np").input_values
+        encoded_sequences_2 = tokenizer(np_speech_inputs, return_tensors="np").input_values
+        for enc_seq_1, enc_seq_2 in zip(encoded_sequences_1, encoded_sequences_2):
+            self.assertTrue(np.allclose(enc_seq_1, enc_seq_2, atol=1e-3))
+
     def test_padding(self, max_length=50):
         def _input_values_have_equal_length(input_values):
             length = len(input_values[0])