Option to build oss faster until we remove these

`controller` and `process_allocator` really are no longer needed to develop oss stuff, but we have to remove important testing, etc. For now, I just to be able to compile locally.

This is a 3x build speed improvement because each of these binaries uses different flags for the underlying crates and causes recompiles.

Differential Revision: [D86880713](https://our.internmc.facebook.com/intern/diff/D86880713/)

**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D86880713/)!

ghstack-source-id: 322762604
Pull Request resolved: #1880

Author: zdevito

setup.py

@@ -161,24 +161,35 @@ def run(self):
     cur = os.environ.get("RUSTFLAGS", "")
     os.environ["RUSTFLAGS"] = (cur + " " + " ".join(flags)).strip()
 
+# Check if MONARCH_BUILD_MESH_ONLY=1 to skip legacy builds
+SKIP_LEGACY_BUILDS = os.environ.get("MONARCH_BUILD_MESH_ONLY", "0") == "1"
 
-rust_extensions = [
-    RustBin(
-        target="process_allocator",
-        path="monarch_hyperactor/Cargo.toml",
-        debug=False,
-    ),
+rust_extensions = []
+
+# Legacy builds (kept for tests that still depend on them)
+if not SKIP_LEGACY_BUILDS:
+    rust_extensions.append(
+        RustBin(
+            target="process_allocator",
+            path="monarch_hyperactor/Cargo.toml",
+            debug=False,
+        )
+    )
+
+# Main extension (always built)
+rust_extensions.append(
     RustExtension(
         "monarch._rust_bindings",
         binding=Binding.PyO3,
         path="monarch_extension/Cargo.toml",
         debug=False,
         features=["tensor_engine"] if USE_TENSOR_ENGINE else [],
         args=[] if USE_TENSOR_ENGINE else ["--no-default-features"],
-    ),
-]
+    )
+)
 
-if USE_TENSOR_ENGINE:
+# Legacy controller bin (kept for tests that still depend on it)
+if USE_TENSOR_ENGINE and not SKIP_LEGACY_BUILDS:
     rust_extensions.append(
         RustExtension(
             {"controller_bin": "monarch.monarch_controller"},