Add nlohmann/json submodule (#80322)
Summary: Introduce nlohmann/json as a submodule within pytorch/third_party. This library is already a transitive dependency and is included in our licenses file. Adding it directly to third_party will enable its use by the CoreML backend.
Test Plan: There are no code changes, so submodule sync and perform the steps outline in the building from source section of the pytorch readme.
Differential Revision: D37449817
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80322
Approved by: https://github.com/mcr229
