Bug-fix: Roberta Embeddings Not Masked
## Summary
I replace the code that makes the position ids with logic closer to the original fairseq `make_positions` function. It wasn't clear to me what to do in the event that the embeddings are passed in directly through `inputs_embeds` so I resorted to the old methodology and just generating a positional id for all inputs.

## Closes
#1761 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=h1) Report
> Merging [#1764](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a80778f40e4738071b5d01420a0328bb00cdb356?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1764/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1764      +/-   ##
==========================================
+ Coverage   79.82%   79.84%   +0.02%     
==========================================
  Files         131      131              
  Lines       19496    19519      +23     
==========================================
+ Hits        15562    15585      +23     
  Misses       3934     3934
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/1764/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3JvYmVydGEucHk=) | `72.57% <100%> (+0.8%)` | :arrow_up: |
| [transformers/tests/modeling\_roberta\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1764/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3JvYmVydGFfdGVzdC5weQ==) | `78.14% <100%> (+2.95%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=footer). Last update [a80778f...3e52915](https://codecov.io/gh/huggingface/transformers/pull/1764?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks! Any way you could do this PR with just the relevant changes (no whitespace changes, no extraneous commits)?<|||||>Hi,

I have pushed just the relevant changes with a single commit.

Many thanks,
Dom<|||||>Thanks! We need to update the TF model as well. (this might be the reason behind the failing unit test, btw)

Will do it unless you have the bandwidth (and the inclination) to do it yourself @DomHudson <|||||>@DomHudson Fixed a bug introduced by this PR's commit + rebased on top of current master.

Please note that for most models, we expect users to define `attention_mask`s themselves. 

If I'm not mistaken, if a user defined an attention_mask that correctly excluded padding, he would not need the padding-aware position_ids from this PR.<|||||>LGTM merging