Make `parallelism` for CircleCI jobs work - but keep it to be `1` for now
# What does this PR do?

Enable `parallelism` for CircleCI jobs. So far I only enable it for torch/tf/flax jobs. It can be switch off easily.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for looking into this. As said internally I'm really not a fan of the split test reports this gets us at the end.

I can probably try to concatenate the failed tests (with details) at the end - although so far I don't have clear idea of the feasibility)

But at least we can switch back to N=1 if we really need to (i.e. we have real difficulty to read the test failures)<|||||>Ready for review again. So far it still uses `parallelism=1` - this PR just provides the necessary change for using `parallelism=N`. Hopefully I can figure out a way to make a better report format in another PR, and we can finally go for N > 1.<|||||>> Thanks! Another thing to look for before enabling it is if we just pay 8x the price for parallelism=8 and not way more than that.

@sgugger I think the expectation to check is if we just pay 1x~2x the price for parallelism=N (say 8). Using 8 executors means less runtime on each executor (ideally 1/8), so the total should be the same (but there is definitely some overhead, like in cache loading / pip install steps)