Cleanup required_torch_version code and references. (#5370)
- Move `required_torch_version` check from deepspeed.runtime.utils to
deepspeed.utils.torch (newly created).
- Remove unused duplicate definition from `tests/unit/util.py`.
- Update all references to this function.
- Switch checks in `deepspeed/runtime/pipe/p2p.py` to use this function.
- Switch checks in `deepspeed/comm/torch.py` to use this function.
---------
Co-authored-by: Lev Kurilenko <113481193+lekurile@users.noreply.github.com>
