Add XuehaiPan to CODEOWNERS for C++ PyTree utilities (pytorch#137408)

Pull Request resolved: pytorch#137408
Approved by: https://github.com/zou3519

Author: XuehaiPan

CODEOWNERS

@@ -183,3 +183,9 @@ torch/cuda/ @eqy @syed-ahmed
 torch/csrc/cuda/ @eqy @syed-ahmed
 torch/backends/cuda/ @eqy @syed-ahmed
 torch/backends/cudnn/ @eqy @syed-ahmed
+
+# PyTree utilities
+/torch/utils/_pytree.py @XuehaiPan
+/torch/utils/_cxx_pytree.py @XuehaiPan
+/torch/utils/pytree/ @XuehaiPan
+/torch/_dynamo/polyfills/pytree.py @XuehaiPan