Add PyObjectSlot member to StorageImpl (#93342)
Part of #91395
Also modifies how `StorageImpl`s are stored in JIT static runtime's `MemoryPlanner`, which used to `std::move` `StorageImpl`s into a vector. But `StorageImpl` can no longer be moved. Instead, `MemoryPlanner` now contains a malloced buffer to which we add new `StorageImpl`s using placement new
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93342
Approved by: https://github.com/ezyang
