[seq2seq] distillation.py accepts trainer arguments
cc @nateraw 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5865?src=pr&el=h1) Report
> Merging [#5865](https://codecov.io/gh/huggingface/transformers/pull/5865?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/529850ae7bca0ff388778c3c0d66240834cf56c3&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5865/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5865?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5865   +/-   ##
=======================================
  Coverage   78.48%   78.48%           
=======================================
  Files         146      146           
  Lines       26200    26200           
=======================================
  Hits        20563    20563           
  Misses       5637     5637           
```



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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5865?src=pr&el=footer). Last update [529850a...259bc29](https://codecov.io/gh/huggingface/transformers/pull/5865?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
