[Summarization Pipeline]: Fix default tokenizer
`pipeline.tokenizer` cannot be a dict!
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4506?src=pr&el=h1) Report
> Merging [#4506](https://codecov.io/gh/huggingface/transformers/pull/4506?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a08652772791fdaeed6f263b1a99926ca64be5dc&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4506/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4506?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4506      +/-   ##
==========================================
- Coverage   77.83%   77.82%   -0.01%     
==========================================
  Files         123      123              
  Lines       20514    20514              
==========================================
- Hits        15968    15966       -2     
- Misses       4546     4548       +2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4506?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/4506/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `76.11% <ø> (ø)` | |
| [src/transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/4506/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9oZl9hcGkucHk=) | `93.06% <0.00%> (-4.96%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4506/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4506/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.83% <0.00%> (+0.32%)` | :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/4506?src=pr&el=footer). Last update [a086527...70d3058](https://codecov.io/gh/huggingface/transformers/pull/4506?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
