aot autograd: consolidate metadata (#96340)
Another bonus of factoring the synthetic_base logic into one place: we used to have a `CompiledRuntimeMetadata` object that encapsulated `ViewAndMutationMeta`, plus a bunch of extra synthetic base metadata that was plumbed around. Now I can kill that first metadata object, and use `ViewAndMutationMeta` on its own everywhere.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96340
Approved by: https://github.com/ezyang
