Revising optional package imports

[laserkelvin]

This PR uses the package registry a little bit more uniformly in determining which optional dependencies have been installed and are available.

• To support Intel XPUs being available in upstream PyTorch now, IPEX is no longer a required dependency and the accelerator and DDP strategies are imported if torch.xpu.is_available() or if IPEX was successfully imported. A warning will be posted if this module is loaded directly and if neither condition is fulfilled.
• On a similar vein, torch_scatter and torch_sparse being unavailable or unimportable would mean every PyG model would not work even if they are not required. This PR adds checks to gracefully try and include those models with explicit torch_scatter and/or torch_sparse dependencies, and warns the user that they will not be available.

[laserkelvin]

The pre-commit workflow fails, because it's not picking up the fact that I'm adding the stuff to __all__ dynamically. I'm fine with this :P