distil-finetuning in run_squad
- Add the option for double loss: fine-tuning + distillation from a larger squad-finetune model.
- Fix `inputs` for `DistilBERT` (also see fix in  `run_glue.py` 702f589848baba97ea4897aa3f0bb937e1ec3bcf)
cf https://github.com/huggingface/transformers/issues/1193#issuecomment-534740929<|||||>Ok, as discussed let's copy this script to the `examples/distillation` folder and keep `run_squad` barebone for now as it's going to evolve in the short term.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1361?src=pr&el=h1) Report
> Merging [#1361](https://codecov.io/gh/huggingface/transformers/pull/1361?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2dc8cb87341223e86220516951bb4ad84f880b4a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1361/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1361?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1361   +/-   ##
=======================================
  Coverage   84.69%   84.69%           
=======================================
  Files          84       84           
  Lines       12596    12596           
=======================================
  Hits        10668    10668           
  Misses       1928     1928
```



------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1361?src=pr&el=footer). Last update [2dc8cb8...b4df865](https://codecov.io/gh/huggingface/transformers/pull/1361?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>no squash @VictorSanh? 😬