Expose padding_strategy on squad processor to fix QA pipeline performance regression
Author: mfuntowicz
Pull request: yes

**Before this PR**:
The squad processor was padding the sequence up to the provided `max_length` parameter which results in a tensor with 512 tokens, mostly padding, making the QA pipeline very slow.

- QA Pipeline (`model="distilbert-base-uncased-distilled-squad"`) = 4.8secs

**After this PR**:
The processor will not be padding at all when coming from the QA pipeline.

- QA Pipeline (`model="distilbert-base-uncased-distilled-squad"`) = 1.29secs

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5932?src=pr&el=h1) Report
> Merging [#5932](https://codecov.io/gh/huggingface/transformers/pull/5932?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d32279438a73e71961f53baa4fb47d0f08c2984d&el=desc) will **increase** coverage by `0.23%`.
> The diff coverage is `30.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5932/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5932?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5932      +/-   ##
==========================================
+ Coverage   78.25%   78.49%   +0.23%     
==========================================
  Files         146      146              
  Lines       26214    26223       +9     
==========================================
+ Hits        20515    20584      +69     
+ Misses       5699     5639      -60     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5932?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.13% <22.22%> (-0.40%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `77.00% <100.00%> (+0.03%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.02% <0.00%> (-69.52%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `43.98% <0.00%> (-49.38%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+0.50%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.60% <0.00%> (+1.16%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.31% <0.00%> (+1.28%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+33.89%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5932/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `95.18% <0.00%> (+74.91%)` | :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/5932?src=pr&el=footer). Last update [d322794...30d41ea](https://codecov.io/gh/huggingface/transformers/pull/5932?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Would be awesome to introduce perf regression testing in this repo too at some point (we have some crude timeouts but maybe something more fine-grained)

Linked issue https://github.com/huggingface/transformers/issues/6144