Valohai integration
Huggingface / Transformers Valohai integration

Changes to existing Transformers code:
- Prints Valohai-styled logs (JSON)

Additional info:
- valohai.yaml has most (but not all) parameters used by run_glue.py
- Valohai execution downloads all glue datas by default (still pretty fast). Download script placed in `utils/download_glue_data.py`.
- Valohai execution only saves model/checkpoint at the end by default (adjust with Valohai UI)
- Valohai execution logs every 25 steps (adjust with Valohai UI)

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=h1) Report
> Merging [#1903](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/0cdfcca24b5739e25b584c05b866baa19ea382ef?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/1903/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1903   +/-   ##
=======================================
  Coverage   84.04%   84.04%           
=======================================
  Files          97       97           
  Lines       14333    14333           
=======================================
  Hits        12046    12046           
  Misses       2287     2287
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=footer). Last update [0cdfcca...66fc8d2](https://codecov.io/gh/huggingface/transformers/pull/1903?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>> * Loss logging no longer accumulates (why was that?) but shows raw loss

With the revised commits, this is no longer true (but I'm not able to edit @JuhaKiili's original message, and he's AFK at Slush :) )

