[sched] polynomial_decay_schedule use default power=1.0
As discussed in https://github.com/huggingface/transformers/pull/6361 we weren't sure why fairseq's `polynomial_decay_schedule` `power` default was `1.0`, and decided to go with `2.0` as the latter does something polynomial.

I got the devs at fairseq to answer this question: https://github.com/pytorch/fairseq/issues/2466#issuecomment-673146603

> myleott wrote:
> This is based on the original BERT code, which implemented a linear decay via a polynomial schedule with power=1.0: https://github.com/google-research/bert/blob/f39e881b169b9d53bea03d2d341b31707a6c052b/optimization.py#L37

So, perhaps we do the same or we don't. 

If we don't - then the doc needs to be fixed that the default is `power=2.0` as currently it says `1.0` - my mistake. If we do (this PR), then the doc is already correct.

Thanks.