Fix RobertaEmbeddings
First of all, the original implementation can define `segment_embeddings` depending on `num_segments` argument. Actually, their model(Roberta) didn't use `segment_embeddings` because they found the effectiveness of `FULL/DOC SENTENCE` setting of inputs. 

And `position_embeddings` should use `padding_idx` to ignore padded inputs. Also the embedding matrix's size should be `padding_idx + max_seq_length + 1`. (e.g If `padding_idx=1` and `max_seq_length=512`, `maxtrix size = (1 + 512 + 1) = 514`.

Last, `position_ids` should be made by considering the previous feature. Below is simple test to make `position_ids` to reflect `padding_idx` of `input_ids`
```
input_ids = torch.randint(0,1000,(3,10))
padding_idx = 0

### dummy padded input
input_ids[:,-2] = padding_idx
input_ids[:,-1] = padding_idx
input_ids[0][-3] = padding_idx
input_ids[-1][-3] = padding_idx

input_ids
>>> tensor([[946, 783, 399, 951, 496, 400, 350,   0,   0,   0],
            [905, 445, 410, 406, 526,   1, 255, 811,   0,   0],
            [815, 669, 813, 708, 475, 232, 190,   0,   0,   0]])

```

```
mask = input_ids.ne(padding_idx).int()
position_ids = (torch.cumsum(mask, dim=1).type_as(mask) * mask).long() + padding_idx

position_ids
>>> tensor([[1, 2, 3, 4, 5, 6, 7, 0, 0, 0],
            [1, 2, 3, 4, 5, 6, 7, 8, 0, 0],
            [1, 2, 3, 4, 5, 6, 7, 0, 0, 0]])

```
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=h1) Report
> Merging [#1052](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/40acf6b52a5250608c2b90edd955835131971d5a?src=pr&el=desc) will **increase** coverage by `0.11%`.
> The diff coverage is `92%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
+ Coverage   79.57%   79.68%   +0.11%     
==========================================
  Files          42       42              
  Lines        6863     6881      +18     
==========================================
+ Hits         5461     5483      +22     
+ Misses       1402     1398       -4
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfYmVydC5weQ==) | `88% <100%> (+0.02%)` | :arrow_up: |
| [pytorch\_transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvbW9kZWxpbmdfcm9iZXJ0YS5weQ==) | `77.51% <91.66%> (+1.62%)` | :arrow_up: |
| [pytorch\_transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX2JlcnQucHk=) | `95.28% <0%> (+0.94%)` | :arrow_up: |
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `74.1% <0%> (+2.87%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=footer). Last update [40acf6b...e2a628a](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1052?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This looks reasonable to me but I'd need to take a better look at it.

Maybe @myleott do you have time to take a quick glance?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
