add a summary report flag for run_examples on CI
Currently, it's hard to derive which example tests were run on CI, and which weren't. Adding `-rA` flag to `pytest`, will now include a summary like:

```
==================================================================== short test summary info =====================================================================
PASSED examples/test_examples.py::ExamplesTests::test_generation
PASSED examples/test_examples.py::ExamplesTests::test_run_glue
PASSED examples/test_examples.py::ExamplesTests::test_run_language_modeling
PASSED examples/test_examples.py::ExamplesTests::test_run_squad
FAILED examples/test_examples.py::ExamplesTests::test_run_pl_glue - AttributeError: 'Namespace' object has no attribute 'gpus'
============================================================ 1 failed, 4 passed, 8 warnings in 42.96s ============================================================
```
which makes it easier to validate whether some example is being covered by CI or not.

The PR came about following on the discussion at https://github.com/huggingface/transformers/pull/6027#issuecomment-663894148
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=h1) Report
> Merging [#6035](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c69ea5efc4eac65b183e8d07b1bf91d20bbe0c8c&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6035/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6035      +/-   ##
==========================================
+ Coverage   78.50%   78.52%   +0.01%     
==========================================
  Files         146      146              
  Lines       26249    26249              
==========================================
+ Hits        20606    20611       +5     
+ Misses       5643     5638       -5     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+1.25%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=footer). Last update [c69ea5e...9bca1bc](https://codecov.io/gh/huggingface/transformers/pull/6035?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
