Don't init TPU device twice
closes #4893

The TPU device was initialized twice when using a the `xla_spawn.py` script. Removing this initialization solves the issue.

@patrickvonplaten, is this necessary for the benchmarking script?
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=h1) Report
> Merging [#4916](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ef2dcdccaa9a115aca44d81f31c6dc4d32bebb3f&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4916/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4916   +/-   ##
=======================================
  Coverage   77.12%   77.13%           
=======================================
  Files         128      128           
  Lines       21650    21649    -1     
=======================================
+ Hits        16698    16699    +1     
+ Misses       4952     4950    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4916/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.19% <100.00%> (+0.69%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=footer). Last update [ef2dcdc...acb4cb4](https://codecov.io/gh/huggingface/transformers/pull/4916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Oh, my bad! Not sure why I left it there<|||||>No worries!