Added nlohmann/json:v3.11.3 as a submodule in ext-src and passed its path into the mscclpp build to avoid downloading the package at build time. (#1330)

[ROCm/rccl commit: b3b0ffdbf3]

projects/rccl/.gitmodules

@@ -2,3 +2,7 @@
 	path = ext-src/mscclpp
 	url = https://github.com/microsoft/mscclpp.git
 	ignore = dirty
+[submodule "ext-src/json"]
+	path = ext-src/json
+	url = https://github.com/nlohmann/json.git
+	ignore = dirty

projects/rccl/cmake/MSCCLPP.cmake

@@ -76,7 +76,7 @@ if(ENABLE_MSCCLPP)
                          # GIT_REPOSITORY      https://github.com/microsoft/mscclpp.git
                          # GIT_TAG             1e82dd444fc1ed8b7add354eebaab8a94e67d5fc
                          INSTALL_DIR         ${MSCCLPP_ROOT}
-                         CMAKE_ARGS          -DGPU_TARGETS=gfx942 -DBYPASS_GPU_CHECK=ON -DUSE_ROCM=ON -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -DBUILD_APPS_NCCL=ON -DBUILD_PYTHON_BINDINGS=OFF -DBUILD_TESTS=OFF -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR> "${CMAKE_PREFIX_PATH_ARG}" "${CMAKE_SHARED_LINKER_FLAGS_INIT_ARG}" "${CMAKE_EXE_LINKER_FLAGS_INIT_ARG}" -DCMAKE_VERBOSE_MAKEFILE=1 "${CMAKE_INSTALL_RPATH_USE_LINK_PATH_ARG}" "${HIP_COMPILER_ARG}"
+                         CMAKE_ARGS          -DGPU_TARGETS=gfx942 -DBYPASS_GPU_CHECK=ON -DUSE_ROCM=ON -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} -DBUILD_APPS_NCCL=ON -DBUILD_PYTHON_BINDINGS=OFF -DBUILD_TESTS=OFF -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR> "${CMAKE_PREFIX_PATH_ARG}" "${CMAKE_SHARED_LINKER_FLAGS_INIT_ARG}" "${CMAKE_EXE_LINKER_FLAGS_INIT_ARG}" -DCMAKE_VERBOSE_MAKEFILE=1 "${CMAKE_INSTALL_RPATH_USE_LINK_PATH_ARG}" "${HIP_COMPILER_ARG}" -DFETCHCONTENT_SOURCE_DIR_JSON=${CMAKE_CURRENT_SOURCE_DIR}/ext-src/json
                          LOG_DOWNLOAD        FALSE
                          LOG_CONFIGURE       FALSE
                          LOG_BUILD           FALSE