CI job cpu (ubuntu-18.04, 3.9, oldest, stable) failing with Segmentation fault

[daniellepintz]

🐛 Bug

The CI job cpu (ubuntu-18.04, 3.9, oldest, stable) has been failing quite consistently on recent PRs with a seg fault:
/home/runner/work/_temp/a06dced7-640c-436b-bbb1-c17e205bf64d.sh: line 2: 3542 Segmentation fault (core dumped) coverage run --source pytorch_lightning -m pytest pytorch_lightning tests -v --durations=50 --junitxml=junit/test-results-Linux-py3.9-oldest-stable.xml

Recent runs:
https://github.com/PyTorchLightning/pytorch-lightning/runs/4597791363?check_suite_focus=true
https://github.com/PyTorchLightning/pytorch-lightning/runs/4598799055?check_suite_focus=true
https://github.com/PyTorchLightning/pytorch-lightning/runs/4599551767?check_suite_focus=true
https://github.com/PyTorchLightning/pytorch-lightning/runs/4589679232?check_suite_focus=true

This job is often the only failing job, making the whole CI red. However this job is not required to merge a PR, so contributors aren't strongly motivated to fix it. As part of #9445 I believe we should either mark this test as required or delete it, with a personal preference for deleting it.

cc @carmocca @akihironitta @Borda

[carmocca]

Duplicate of #11184