[wav2vec2 / Deepspeed] sync LayerDrop for Wav2Vec2Encoder + tests
This PR continues https://github.com/huggingface/transformers/pull/11638 and:
- adds the same gpu syncing for `Wav2Vec2Encoder` LayerDrop as there is for ` Wav2Vec2EncoderStableLayerNorm`
- double the tests to test `"patrickvonplaten/wav2vec2_tiny_random"` to exercise `Wav2Vec2Encoder` too

@patrickvonplaten 
