disable concat nested namespace check (pytorch#49571)

Summary:
Pull Request resolved: pytorch#49571

Disable nested namespace check since OSS standard is
```
set(CMAKE_CXX_STANDARD 14)
```
and its currently causing confusion on clang-tidy internally such as D25214452

Test Plan: clang-tidy

Reviewed By: xuzhao9

Differential Revision: D25626392

fbshipit-source-id: 1fb472c89ebe9b83718ae27f2c1d77b8b2412b5e

Author: Rong Rong (AI Infra)

.clang-tidy

@@ -22,6 +22,7 @@ cppcoreguidelines-*,
 hicpp-exception-baseclass,
 hicpp-avoid-goto,
 modernize-*,
+-modernize-concat-nested-namespaces,
 -modernize-return-braced-init-list,
 -modernize-use-auto,
 -modernize-use-default-member-init,