Loading preprocessed Wikipedia dataset requires apache_beam
Running 

`nlp.load_dataset("wikipedia", "20200501.en", split="train", dir="/tmp/wikipedia")`

gives an error if apache_beam is not installed, stemming from

https://github.com/huggingface/nlp/blob/38eb2413de54ee804b0be81781bd65ac4a748ced/src/nlp/builder.py#L981-L988

This succeeded without the dependency in version 0.3.0. This seems like an unnecessary dependency to process some dataset info if you're using the already-preprocessed version. Could it be removed?