[PyTorch CCA] Refactor caching allocator config code (#110123)
Summary: This diff refactors the code by moving CUDAAllocatorConfig into the header file. This config refactoring is done so that we can use the same config code for CUDA pinned memory as well.
Test Plan: sandcastle
Differential Revision: D49653265
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110123
Approved by: https://github.com/zdevito
