[BE] Delete torch._dl extension (pytorch#84361)

And lots of complexity around the availability of RTLD_GLOBAL flags in `os` module
As this flag is always present since Python-3.3, see https://docs.python.org/3/library/os.html#os.RTLD_GLOBAL

Fixes pytorch#84351

Pull Request resolved: pytorch#84361
Approved by: https://github.com/kit1980

setup.py

@@ -873,12 +873,6 @@ def make_relative_rpath_args(path):
     extensions.append(C)
     extensions.append(C_flatbuffer)
 
-    if not IS_WINDOWS:
-        DL = Extension("torch._dl",
-                       sources=["torch/csrc/dl.c"],
-                       language='c')
-        extensions.append(DL)
-
     # These extensions are built by cmake and copied manually in build_extensions()
     # inside the build_ext implementation
     if cmake_cache_vars['BUILD_CAFFE2']:

torch/__init__.py

@@ -171,20 +171,11 @@ def _load_global_deps():
     # you load consistently use the same libstdc++, or you may have
     # mysterious segfaults.
     #
-    import os as _dl_flags
-    if not hasattr(_dl_flags, 'RTLD_GLOBAL') or not hasattr(_dl_flags, 'RTLD_LAZY'):
-        try:
-            # next try if DLFCN exists
-            import DLFCN as _dl_flags  # type: ignore[import, no-redef]
-        except ImportError:
-            # as a last attempt, use compile-time constants
-            import torch._dl as _dl_flags  # type: ignore[import, no-redef]
     old_flags = sys.getdlopenflags()
-    sys.setdlopenflags(_dl_flags.RTLD_GLOBAL | _dl_flags.RTLD_LAZY)
+    sys.setdlopenflags(os.RTLD_GLOBAL | os.RTLD_LAZY)
     from torch._C import *  # noqa: F403
     sys.setdlopenflags(old_flags)
     del old_flags
-    del _dl_flags
 
 else:
     # Easy way.  You want this most of the time, because it will prevent