add a summary report flag for run_examples on CI (#6035)

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.

.circleci/config.yml

@@ -73,7 +73,7 @@ jobs:
             - checkout
             - run: sudo pip install .[sklearn,torch,testing]
             - run: sudo pip install -r examples/requirements.txt
-            - run: python -m pytest -n 8 --dist=loadfile -s ./examples/ | tee output.txt
+            - run: python -m pytest -n 8 --dist=loadfile -rA -s ./examples/ | tee output.txt
             - store_artifacts:
                   path: ~/transformers/output.txt
                   destination: test_output.txt