Fix onnx export input names order
This PR makes it possible to export custom bert models to onnx.

It resolves the issue addressed [here](https://github.com/huggingface/transformers/issues/4523#issuecomment-634920569).

TODO: 
- [x] update ensure_valid_input function
- [x] update test ensure_valid_input_function
- [x] update convert_pytorch
- [x] add a test of exporting a custom pytorch models


@mfuntowicz
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4641?src=pr&el=h1) Report
> Merging [#4641](https://codecov.io/gh/huggingface/transformers/pull/4641?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/14cb5b35faeda7881341656aacf89d12a8a7e07b&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4641/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4641?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4641      +/-   ##
==========================================
- Coverage   78.04%   78.01%   -0.03%     
==========================================
  Files         123      123              
  Lines       20477    20477              
==========================================
- Hits        15981    15975       -6     
- Misses       4496     4502       +6     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4641?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/4641/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcGkucHk=) | `93.06% <0.00%> (-4.96%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4641/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4641?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/transformers/pull/4641?src=pr&el=footer). Last update [14cb5b3...a4d4611](https://codecov.io/gh/huggingface/transformers/pull/4641?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>@mfuntowicz what do you think of my proposed changes?

I'm not sure why the CI fails on `TFAutoModelTest.test_from_identifier_from_model_type` as I haven't touched it.<|||||>LGTM! Thanks @RensDimmendaal for looking at this :)<|||||>@LysandreJik should we merge with the failing test? It seems totally unrelated 