[pipeline] Tokenizer should not add special tokens for text generation
This PR fixes generation in pipelines for all models whose tokenizer adds special tokens to the input, *e.g.* XLNet. 

I think this is good for now, but I think the `_parse_and_tokenize()` function needs a larger refactoring to allow more flexibility in the future, also see Issue: #4501 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=h1) Report
> Merging [#4686](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9c17256447b91cf8483c856cb15e95ed30ace538&el=desc) will **increase** coverage by `0.24%`.
> The diff coverage is `75.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4686/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4686      +/-   ##
==========================================
+ Coverage   77.23%   77.47%   +0.24%     
==========================================
  Files         128      128              
  Lines       21050    21051       +1     
==========================================
+ Hits        16257    16309      +52     
+ Misses       4793     4742      -51     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `76.15% <75.00%> (+0.04%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.17% <0.00%> (+0.23%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.78% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4686/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.21% <0.00%> (+14.10%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=footer). Last update [9c17256...95cf209](https://codecov.io/gh/huggingface/transformers/pull/4686?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
