[FSDP] Fix exec order validation (static variable issue)
This fixes a bad mistake on my end from https://github.com/pytorch/pytorch/pull/75902.
I unintentionally made the variables in `_ExecOrderData` static variables instead of instance variables, so multiple FSDP models run in the same process share the same `_ExecOrderData`, which is clearly incorrect. This PR changes them to instance variables.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76273
Approved by: https://github.com/rohan-varma
