Updating schedules for state_dict saving/loading
This PR updates the schedules so that they can be saved/reloaded using the standard `state_dict()` and `load_state_dict()` methods of PyTorch [`LambdaLR` model](https://pytorch.org/docs/stable/optim.html#torch.optim.lr_scheduler.LambdaLR.load_state_dict).

Useful for continuing stopped training as mentioned in #839
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=h1) Report
> Merging [#872](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/268c6cc160ba046d6a91747c5f281f82bd88a4d8?src=pr&el=desc) will **increase** coverage by `0.12%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
+ Coverage    78.9%   79.03%   +0.12%     
==========================================
  Files          34       34              
  Lines        6192     6228      +36     
==========================================
+ Hits         4886     4922      +36     
  Misses       1306     1306
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tests/optimization\_test.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdGVzdHMvb3B0aW1pemF0aW9uX3Rlc3QucHk=) | `98.97% <100%> (+0.4%)` | :arrow_up: |
| [pytorch\_transformers/optimization.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvb3B0aW1pemF0aW9uLnB5) | `96.62% <100%> (+0.33%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=footer). Last update [268c6cc...0740e63](https://codecov.io/gh/huggingface/pytorch-transformers/pull/872?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
