updated hans eval instructions
I've updated the info regarding how hans evaluation can be carried out. 

I've also renamed `run_hans.py` to `test_hans.py` to restore the previous file convention and also indicate that HANS only supports evaluation.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5083?src=pr&el=h1) Report
> Merging [#5083](https://codecov.io/gh/huggingface/transformers/pull/5083?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7291ea0bff57a017e71b1ea8ec01ff19da298bf0&el=desc) will **decrease** coverage by `0.39%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5083/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5083?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5083      +/-   ##
==========================================
- Coverage   77.24%   76.84%   -0.40%     
==========================================
  Files         133      133              
  Lines       22146    22146              
==========================================
- Hits        17107    17019      -88     
- Misses       5039     5127      +88     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5083?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/5083/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl94bG0ucHk=) | `56.09% <0.00%> (-19.76%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5083/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.71% <0.00%> (-0.94%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5083/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.61% <0.00%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5083?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/5083?src=pr&el=footer). Last update [7291ea0...b485684](https://codecov.io/gh/huggingface/transformers/pull/5083?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
