Truncation in GLUE should be longest first
The GLUE example currently crashes with the QQP task because of the truncation.

It outputs the following warnings:

```
ERROR:transformers.tokenization_utils:We need to remove 186 to truncate the inputbut the first sequence has a length 25. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
ERROR:transformers.tokenization_utils:We need to remove 49 to truncate the inputbut the first sequence has a length 34. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
ERROR:transformers.tokenization_utils:We need to remove 203 to truncate the inputbut the first sequence has a length 42. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
ERROR:transformers.tokenization_utils:We need to remove 39 to truncate the inputbut the first sequence has a length 28. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
ERROR:transformers.tokenization_utils:We need to remove 23 to truncate the inputbut the first sequence has a length 20. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
ERROR:transformers.tokenization_utils:We need to remove 91 to truncate the inputbut the first sequence has a length 63. Please select another truncation strategy than TruncationStrategy.ONLY_FIRST, for instance 'longest_first' or 'only_second'.
```

before crashing with the following:

```
ValueError: expected sequence of length 128 at dim 1 (got 202)
```

closes https://github.com/huggingface/transformers/issues/5460
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5472?src=pr&el=h1) Report
> Merging [#5472](https://codecov.io/gh/huggingface/transformers/pull/5472?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/306f1a269504b781f886d75105acabf8ae95bd11&el=desc) will **decrease** coverage by `1.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5472/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5472?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5472      +/-   ##
==========================================
- Coverage   77.86%   76.77%   -1.09%     
==========================================
  Files         141      141              
  Lines       24608    24608              
==========================================
- Hits        19160    18892     -268     
- Misses       5448     5716     +268     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5472?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `49.09% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `23.62% <0.00%> (-73.11%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `26.92% <0.00%> (-68.47%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.43% <0.00%> (ø)` | |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.09% <0.00%> (+1.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `98.37% <0.00%> (+25.00%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5472/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `94.98% <0.00%> (+74.19%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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