Update Adam optimizer to follow pytorch convention for betas parameter (#510)
see #510

Update optimiser to follow pytorch convention ([Adam Optimiser](https://pytorch.org/docs/stable/optim.html#torch.optim.Adam)) instead of tensorflow, to allow for better integration with other pytorch libraries and frameworks.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?src=pr&el=h1) Report
> Merging [#723](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/98dc30b21e3df6528d0dd17f0910ffea12bc0f33?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `83.33%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #723   +/-   ##
=======================================
  Coverage   62.22%   62.22%           
=======================================
  Files          18       18           
  Lines        3979     3979           
=======================================
  Hits         2476     2476           
  Misses       1503     1503
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/optimization.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvb3B0aW1pemF0aW9uLnB5) | `74.26% <100%> (ø)` | :arrow_up: |
| [pytorch\_pretrained\_bert/optimization\_openai.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvb3B0aW1pemF0aW9uX29wZW5haS5weQ==) | `34.84% <66.66%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?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-pretrained-BERT/pull/723?src=pr&el=footer). Last update [98dc30b...c988590](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/723?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok, can you update the examples that use these optimizers as well?<|||||>> Ok, can you update the examples that use these optimizers as well?

I had a look at the examples, all seem to use the default values for b1/b2 so there shouldn't be any change required.<|||||>Perfect!