allow changing the cuda allocator settings even after the process started (#84970)
Summary:
- expose a python call to set the allocator settings, it uses the same format as the value for PYTORCH_CUDA_ALLOCATOR
- keep the implementation contained within the cpp file to avoid increasing build times, only expose a function to call the setting
- make some of the Allocator Config methods public, now it looks more like a singleton
Test Plan: added the unit test
Differential Revision: D39487522
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84970
Approved by: https://github.com/zdevito
