New SQuAD API for distillation script
The squad distillation script is still using methods from files that do not exist anymore (utils_squad and utils_squad_evaluate).

I updated the script to use the newer API.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2457?src=pr&el=h1) Report
> Merging [#2457](https://codecov.io/gh/huggingface/transformers/pull/2457?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/16ce15ed4bd0865d24a94aa839a44cf0f400ef50?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/2457/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2457?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2457   +/-   ##
=======================================
  Coverage   73.24%   73.24%           
=======================================
  Files          87       87           
  Lines       15001    15001           
=======================================
  Hits        10988    10988           
  Misses       4013     4013
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2457?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/2457?src=pr&el=footer). Last update [16ce15e...8eaea4e](https://codecov.io/gh/huggingface/transformers/pull/2457?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
