Install c10d headers with absolute path (#86257)
https://github.com/pytorch/pytorch/pull/85780 updated all c10d headers in pytorch to use absolute path following the other distributed components. However, the headers were still copied to `${TORCH_INSTALL_INCLUDE_DIR}/torch`, thus external extentions still have to reference the c10d headers as `<c10d/*.h>`, making the usage inconsistent (the only exception was c10d/exception.h, which was copied to `${TORCH_INSTALL_INCLUDE_DIR}/torch/csrc/distributed/c10d`).
This patch fixes the installation step to copy all c10d headers to `${TORCH_INSTALL_INCLUDE_DIR}/torch/csrc/distributed/c10d`, thus external extensions can consistently reference c10d headers with the absolute path.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86257
Approved by: https://github.com/kumpera
