Swap of optimizer.step and scheduler.step for lm finetuning examples
Prior to PyTorch 1.1.0, the learning rate scheduler was expected to be called before the optimizer’s update; 1.1.0 changed this behavior in a BC-breaking way. If you use the learning rate scheduler (calling scheduler.step()) before the optimizer’s update (calling optimizer.step()), this will skip the first value of the learning rate schedule. If you are unable to reproduce results after upgrading to PyTorch 1.1.0, please check if you are calling scheduler.step() at the wrong time.

This is my first time very simple PR, please correct me if there's anything done wrong xD.

[link](https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate)
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1056?src=pr&el=h1) Report
> Merging [#1056](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1056?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/c589862b783b94a8408b40c6dc9bf4a14b2ee391?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1056/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1056?src=pr&el=tree)

```diff
@@          Coverage Diff           @@
##           master   #1056   +/-   ##
======================================
  Coverage    79.6%   79.6%           
======================================
  Files          42      42           
  Lines        6863    6863           
======================================
  Hits         5463    5463           
  Misses       1400    1400
```



------

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<|||||>Looks good to me, thanks @Morizeyao!