big cpp test reorg (#24801)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24801
This is to fix the ODR-violations in fbcode static builds, which have been broken for several months.
This PR is unfortunately quite large, but the changes are only mechanical:
1. Tests defined in header files -> tests defined in cpp files
2. Remove the `torch::jit::testing` namespace -> `torch::jit`.
3. Single `test.h` file that aggregates all tests.
4. Separate out files for gtest and python versions of the tests instead of using a build flag
5. Add a readme for how to add a new test, and explaining a bit about why the cpp tests are the way they are.
Test Plan: Imported from OSS
Differential Revision: D16878605
Pulled By: suo
fbshipit-source-id: 27b5c077dadd990a5f74e25d01731f9c1f491603
