BatchFeature should cast to `np.float32` by default
Currently the default dtype for Speech Feature Extractors is `numpy.float64` which leads to two problems:

1) It makes the data processing extremely expensive for the RAM. Many sound formats are stored in int16 (such as `.wav`) and are then transformed to float64 which unnecessarly increases RAM by a factor of 4. We should at least stick to `float32`
2) Currently we have added some hacks to the Wav2Vec2 and Speech2TextTransformer feature extractors to prevent Double vs. Float dtype mismatches: https://github.com/huggingface/transformers/blob/f6e254474cb4f90f8a168a599b9aaf3544c37890/src/transformers/models/wav2vec2/feature_extraction_wav2vec2.py#L87

The main problem is that `np.asarray([....])` by default creates a np.float64 array and that we just pass that format along.
=> We should either always cast to float32 in BatchFeature (see here: https://github.com/huggingface/transformers/blob/f6e254474cb4f90f8a168a599b9aaf3544c37890/src/transformers/feature_extraction_utils.py#L151) or add a flag `dtype` to BatchFeature.

@patrickvonplaten 