[pt] move csrc shm logic to aten storage utils (#95228)
Summary:
This is part 1 of the effort to support `share_memory_()` in C++ aten library.
This allows C++ code to in place replace the tensor storage to shm based.
For now fd based shm is the only implementation supported to simplify memory management in general.
This first part intentionally avoids public api changes (to `TensorBase`, see comments in `StorageUtil.h`) such that we can get the core features usable outside pt/csrc first. The API addition to `Tensor` or `TensorBase` would involve more distracting changes and make the change harder to review.
Test Plan:
```
buck test caffe2:StorageUtils_test
```
Differential Revision: D43467616
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95228
Approved by: https://github.com/ezyang
