[pybind] Fix rpath for custom ops (#7153)

* [pybind] Fix rpath for custom ops

Summary: This is a followup to #7096. That PR was aiming to rely on
`CMAKE_INSTALL_RPATH_USE_LINK_PATH` to set RPATH automatically. However
due to the caveat that we are not installing `_portable_lib.so` into the
correct path in CMake stage (we move the .so in setup.py after it's
installed), we are not able to add the correct RPATH to
libcustom_ops_aot_lib.so.

This PR still set the INSTALL_RPATH manually and adds TODOs to fix it
later.

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:

* Remove typo

Summary:

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags: