[Sequence Feature Extraction] Add truncation

[patrickvonplaten]

What does this PR do?

This PR adds truncation to speech-related feature extractors. It should enable use cases such as: #12774

Different to our tokenizers we allow truncation to be just True or False => there is no "truncation strategy". The reason is that for feature extractors the input cannot be a "pair" of input sequences so there is essentially just one use case for truncation that applies to all inputs.

The logic is equivalent to Tokenizers with a small exception in error handling. The differences are shown in example 1. & 2. in the following:

1. truncation=True, no padding, no max_length. IMO this should be an error because it's not clear what should be done here. In Tokenizers we don't throw an error, but simply don't do anything. Throwing an error is better here IMO

from transformers import Wav2Vec2FeatureExtractor, BatchFeature
feat_extractor = Wav2Vec2FeatureExtractor()
dummy_inputs = BatchFeature({"input_values": [[0.1, 0.2, 0.3], [0.1]]})

feat_extractor.pad(dummy_inputs, truncation=True) # -> throws error since `max_length` is not defined 

2. truncation=True, "longest" padding, no max_length. IMO this should be an error because it doesn't make sense to "truncate" and to pad to the longest tensor in the batch. If we pad to the longest batch, we can't truncate. In Tokenizers we don't throw an error, but simply don't do anything. Throwing an error is better here IMO

from transformers import Wav2Vec2FeatureExtractor, BatchFeature
feat_extractor = Wav2Vec2FeatureExtractor()
dummy_inputs = BatchFeature({"input_values": [[0.1, 0.2, 0.3], [0.1]]})

feat_extractor.pad(dummy_inputs, truncation=True, padding="longest") # -> throws error since `max_length` is not defined and padding not "max_length"

3. truncation=True, "max_length" padding. Here the logic is equivalent to tokenizers

from transformers import Wav2Vec2FeatureExtractor, BatchFeature
feat_extractor = Wav2Vec2FeatureExtractor()
dummy_inputs = BatchFeature({"input_values": [[0.1, 0.2, 0.3], [0.1]]})

feat_extractor.pad(dummy_inputs, truncation=True, max_length=2, padding="max_length") # -> output shape is [2, 2]

In addition pad_to_multiple_of works correctly and equivalent to the Tokenizers. Tests are added to cover all possible use cases.

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[sgugger]

Thanks for adding this new feature! My main comment is on the default for truncation (why have None if it's just a True/False flag), the rest are just nits.

[LysandreJik]

LGTM! Great tests.