refactor(tests): migrate to device-parametrized TEST_P infrastructure

Replace TEST_F with TEST_P across all test suites so each suite runs on
both CPU and CUDA without duplicating test logic. Adds InfiniTrainTestP,
TensorTestBaseP, AutogradTestBaseP, and DistributedInfiniTrainTestP base
classes with automatic CUDA/NCCL skip guards. Introduces
INFINI_TRAIN_REGISTER_TEST* C++ macros and infini_train_add_test_suite
CMake macro to eliminate repetitive INSTANTIATE_TEST_SUITE_P /
infini_train_add_test boilerplate. Removes deprecated test/, slow/, and
split optimizer test files; consolidates optimizer tests into a single
binary with creation  + step suites.

Author: chen2021673

.gitmodules

@@ -1,3 +1,6 @@
+[submodule "third_party/googletest"]
+	path = third_party/googletest
+	url = https://github.com/google/googletest.git
 [submodule "third_party/glog"]
 	path = third_party/glog
 	url = https://github.com/google/glog.git

CMakeLists.txt

@@ -4,7 +4,7 @@ option(USE_CUDA "Support NVIDIA CUDA" OFF)
 option(PROFILE_MODE "ENABLE PROFILE MODE" OFF)
 option(USE_OMP "Use OpenMP as backend for Eigen" ON)
 option(USE_NCCL "Build project for distributed running" ON)
-option(BUILD_TEST "Build InfiniTrain tests" ON)
+option(BUILD_TEST "Build InfiniTrain tests" OFF)
 
 project(infini_train VERSION 0.5.0 LANGUAGES CXX)
 
@@ -16,17 +16,15 @@ set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 # ------------------------------------------------------------------------------
-# GoogleTest (FetchContent)
+# GoogleTest (submodule)
 # ------------------------------------------------------------------------------
 if(BUILD_TEST)
-  include(FetchContent)
-  FetchContent_Declare(
-    googletest
-    GIT_REPOSITORY https://github.com/google/googletest.git
-    GIT_TAG v1.14.0
-  )
+  if(NOT EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/third_party/googletest/CMakeLists.txt)
+    message(FATAL_ERROR "googletest submodule not found at third_party/googletest. "
+      "Run: git submodule update --init third_party/googletest")
+  endif()
   set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
-  FetchContent_MakeAvailable(googletest)
+  add_subdirectory(third_party/googletest)
   enable_testing()
 endif()