Add python registration support for IREE dialects. (#13549)

Unlike the previous mechanism, this will automatically register all
known dialects with any context created from python.

Fixes #13477 after #13395 removed an API that was load bearing to a
customer use case.
diff --git a/compiler/setup.py b/compiler/setup.py
index f52b20f..df9e1e9 100644
--- a/compiler/setup.py
+++ b/compiler/setup.py
@@ -408,6 +408,7 @@
         # it also needs to be enabled on the build side.
         # CMakeExtension("iree.compiler._mlir_libs._mlirHlo"),
         CMakeExtension("iree.compiler._mlir_libs._mlirLinalgPasses"),
+        CMakeExtension("iree.compiler._mlir_libs._site_initialize_0"),
     ],
     cmdclass={
         "build": CustomBuild,