Add DistilHuBERT 
# What does this PR do?

This adds the DistilHuBERT conversion script and integration tests.

Paper: [DistilHuBERT: Speech Representation Learning by Layer-wise Distillation of Hidden-unit BERT](https://arxiv.org/abs/2110.01900)

The original `s3prl` implementation and weights: https://github.com/s3prl/s3prl/tree/master/s3prl/upstream/distiller
Feel free to merge after fixing the last tests