[mbart] Fix fp16 testing logic
the expected logits must be in the same dtype as the resulting logits.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=h1) Report
> Merging [#4949](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/473808da0d476792070f0e7dfebcf1121a12a34f&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4949/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4949   +/-   ##
=======================================
  Coverage   77.14%   77.14%           
=======================================
  Files         128      128           
  Lines       21745    21745           
=======================================
  Hits        16775    16775           
  Misses       4970     4970           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.26% <0.00%> (-0.16%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4949/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.79% <0.00%> (+0.40%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=footer). Last update [473808d...a9ebdd5](https://codecov.io/gh/huggingface/transformers/pull/4949?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
