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

test/cpp/jit/torch_python_test.cpp

@@ -0,0 +1,28 @@
+#include <test/cpp/jit/tests.h>
+#include <c10/util/Exception.h>
+
+namespace torch {
+namespace jit {
+
+#if defined(_WIN32)
+void runJITCPPTests(bool runCuda) {
+  TORCH_INTERNAL_ASSERT("JIT tests not yet supported on Windows");
+}
+#else
+#define JIT_TEST(name) test##name();
+TORCH_API void runJITCPPTests(bool runCuda) {
+  TH_FORALL_TESTS(JIT_TEST)
+  if (runCuda) {
+    TH_FORALL_TESTS_CUDA(JIT_TEST)
+  }
+
+  // This test is special since it requires prior setup in python.
+  // So it is not part of the general test list (which is shared between the gtest
+  // and python test runners), but is instead invoked manually by the
+  // torch_python_test.cpp
+  testEvalModeForLoadedModule();
+}
+#undef JIT_TEST
+#endif
+} // namespace jit
+} // namespace torch