Remove cached torch_extensions on CI runners
# What does this PR do?

- The test
  ```
  tests/deepspeed/test_deepspeed.py::TrainerIntegrationDeepSpeed::test_hf_scheduler_ds_optimizer
  ```
  failed since 2 weeks due to some cache issue. The error message is
  ```bash
  E   ImportError: /github/home/.cache/torch_extensions/py38_cu113/fused_adam/fused_adam.so: undefined symbol: _ZN3c104impl8GPUTrace13gpuTraceStateE`
  ```

- After I remove the cache (on the host runners, not inside the running docker) by
  ```bash
  sudo rm -rf /home/github_actions/actions-runner/_work_temp/_github_home/.cache/torch_extensions/py38_cu113/
  ```
  the test passes.

- This PR add the following in the workflow file
  ```bash
  rm -rf /github/home/.cache/torch_extensions/
  ```
  to avoid the same problem occurring in the future.

Remark: Notice the host directory
```
/home/github_actions/actions-runner/_work_temp/_github_home/
```
is mapped to 
```
 /github/home/
```
inside the running docker (we can see this in the job run page).
_The documentation is not available anymore as the PR was closed or merged._