Fix Bug in FlaxWav2Vec2 Slow Test
This PR fixes a small bug the `test_inference_ctc_robust_batched` FlaxWav2Vec2 slow test. Currently, the input processor is run with `return_tensors="pt"` and `trunctation=True`:
https://github.com/huggingface/transformers/blob/05c237ea94e08786abbac6c6185cfdfa262a8c53/tests/wav2vec2/test_modeling_flax_wav2vec2.py#L387
However, as outlined in `feature_extraction_sequence_utils.py`, when setting `trunctation=True`, one must **also** specify the `max_len`:
https://github.com/huggingface/transformers/blob/6e57a56987ff201747f5f01bbce3ed2c0fda1910/src/transformers/feature_extraction_sequence_utils.py#L326-L327
The modifications make the change to `return_tensors="np"` and remove the `truncation` flag. The first change returns the native tensor type for Flax (`np` as opposed to `pt`), and the second change aligns the test with its PyTorch counterpart:
https://github.com/huggingface/transformers/blob/6e57a56987ff201747f5f01bbce3ed2c0fda1910/tests/wav2vec2/test_modeling_wav2vec2.py#L1170


 