[Examples] Generalise run audio classification for log-mel models
# What does this PR do?

Currently, `run_audio_classification.py` hard-codes the model input name to `input_values` in the pre-processing function. This makes it compatible with Wav2Vec2-style CTC models, but not other speech models that use log-mel `input_features` (e.g. Whisper or AST).

We adopt the same strategy that we use in `run_speech_recognition_seq2seq.py` and set this to the correct model input name (based on the feature extractor's attribute `.model_input_names`):
https://github.com/huggingface/transformers/blob/1d4b79785263077f9f09ddde5a75ae4f116e85d7/examples/pytorch/speech-recognition/run_speech_recognition_seq2seq.py#L419
_The documentation is not available anymore as the PR was closed or merged._