[monarch/ext] fold monarch_meta_extension into actor_extension

Pull Request resolved: #463

Right now we have 3 extensions:
- actor_extension
- monarch_extension (will be turned into tensor_engine_extension)
- monarch_meta_extension (internal-only).

This diff folds `monarch_meta_extension` into `actor_extension` to reduce the number of independent shared objects, using condition compilation to make the OSS build work.
ghstack-source-id: 294947045
@exported-using-ghexport

Differential Revision: [D77941251](https://our.internmc.facebook.com/intern/diff/D77941251/)

Author: suo

actor_extension/src/lib.rs

@@ -16,6 +16,9 @@ mod blocking;
 mod code_sync;
 mod panic;
 
+#[cfg(fbcode_build)]
+mod meta;
+
 fn get_or_add_new_module<'py>(
     module: &Bound<'py, PyModule>,
     module_name: &str,
@@ -113,5 +116,8 @@ pub fn mod_init(module: &Bound<'_, PyModule>) -> PyResult<()> {
     crate::panic::register_python_bindings(&get_or_add_new_module(module, "panic")?)?;
 
     crate::blocking::register_python_bindings(&get_or_add_new_module(module, "blocking")?)?;
+
+    #[cfg(fbcode_build)]
+    crate::meta::register_python_bindings(&get_or_add_new_module(module, "meta")?)?;
     Ok(())
 }