[SPMD] Implement split_fused_optimizer to split one fused_optimizer node to two (#98784)
Several optimization passes requires the ability to split the fused_optimizer. This PR adds the API to support the use cases.
Differential Revision: [D44806450](https://our.internmc.facebook.com/intern/diff/D44806450/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98784
Approved by: https://github.com/mrshenli
