[CI] clear `~/.cache/torch_extensions` between builds
This PR is trying to address CI failures with pt-nightly. https://github.com/huggingface/transformers/runs/4280926354?check_suite_focus=true

`~/.cache/torch_extensions/` currently uses a single hardcoded path to install all custom cuda extensions and so when it was built with pt-1.8 but then attempted to be used with pt-nightly (pt-1.11-to-be), the following happens:

```
ImportError: /github/home/.cache/torch_extensions/py38_cu111/cpu_adam/cpu_adam.so: undefined symbol: curandCreateGenerator
``` 

pt-1.10 has improved the situation by adding a prefix: `~/.cache/torch_extensions/py38_cu113` which makes the builds not shared between different cuda and python versions, but it missed the crucial pt-version in that prefix. I reported the issue here:
https://github.com/pytorch/pytorch/issues/68905

And of course ideally all the builds should be installed into the virtual python environment and not have a global shared dir.

This PR tries to address the issue by wiping out `~/.cache/torch_extensions/` completely when CI starts.

This of course means `deepspeed` will rebuild the extensions on every CI run, but this is actually a good thing, because then we really test the right version of it. It does it really fast so it shouldn't introduce a large overhead.

@LysandreJik 