Fix encode plus
Fixing the tensor creation in encode_plus
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=h1) Report
> Merging [#2148](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/030faccb8d45be9bdd2b4b80ff26f36dc41f622a?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `30.76%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2148/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2148      +/-   ##
==========================================
- Coverage   80.07%   80.05%   -0.03%     
==========================================
  Files         112      112              
  Lines       16866    16868       +2     
==========================================
- Hits        13505    13503       -2     
- Misses       3361     3365       +4
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2148/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl91dGlscy5weQ==) | `90.22% <30.76%> (-0.8%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=footer). Last update [030facc...3d57c51](https://codecov.io/gh/huggingface/transformers/pull/2148?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great! Nice catch @LysandreJik 