[PyTorch] Add comment to unify macro and rename one macro (#52573)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52573
## Summary
Address comments in https://github.com/pytorch/pytorch/pull/52540
1. Add a comment to indicate that the macros `BUILD_LITE_INTERPRETER` and `C10_MOBILE` will be unified.
2. Rename the macro `DBUILD_LITE_INTERPRETER` to `BUILD_LITE_INTERPRETER`
## Test plan
1. `MACOSX_DEPLOYMENT_TARGET=10.9 CC=clang CXX=clang++ USE_CUDA=0 DEBUG=1 MAX_JOBS=16 BUILD_LITE_INTERPRETER=1 python setup.py develop`
2. `/Users/chenlai/pytorch/cmake-build-debug/bin/test_lite_interpreter_runtime --gtest_filter=* --gtest_color=no`
Test Plan: Imported from OSS
Reviewed By: iseeyuan
Differential Revision: D26572742
Pulled By: cccclai
fbshipit-source-id: c8895fcfe8dd893f8157913f110e2ba025fc3955
