[PyTorch] Reference to c10::GetCPUAllocator() directly (#49068)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49068
TH folder has some kernal implementations referenced by ATen/native. It goes with ATen/native in the follow-up diff for per-app selective build. ATen/Context.cpp stays in the lib level and should not reference to symbols in TH directly.
It's a simple change in this diff, as ```getTHDefaultAllocator()``` did nothing but returns ```c10::GetCPUAllocator()```. Use ```c10::GetCPUAllocator()``` instead of going extra route through ```getTHDefaultAllocator()```.
ghstack-source-id: 118151905
Test Plan: CI
Reviewed By: dhruvbird
Differential Revision: D24147914
fbshipit-source-id: 37efb43adc9b491c365df0910234fa6a8a34ec25
