make monarch build compatible for both conda and uv

Summary:
## The Problem
Basically currently the monarch wheel cannot be installed by `uv`:
```
ImportError: libpython3.10.so.1.0: cannot open shared object file: No such file or directory
```

## Root Cause
The root cause is 
- monarch wheel is built in Conda environment with rpaths baked into the compiled `.so` files
- `uv` installs Python in a different location (`~/.local/share/uv/python/...`), and therefore the baked-in paths don't exist in uv environments

## Proper Fix 
There are workaround for `uv` installation, e.g.  set `LD_LIBRARY_PATH` to point to `uv`'s Python library directory, but I think we should do a proper fix to the monarch build since ChatGPT said "Best practice is not to link extensions against libpython on Unix; the loader resolves Python symbols from the running interpreter. PyO3’s extension-module feature exists to avoid linking to libpython altogether. Manylinux guidance and PyO3 docs echo this." https://github.com/pypa/manylinux/issues/69?utm_source=chatgpt.com

This diff is an attempt to use ` extension-module` feature and roughly does two things
* Turn on the feature (in BUCK)
* Remove the hardcoded `rpath` from `setup.py`

-----
## Appendix

Workaround by setting `LD_LIBRARY_PATH`

1. Find your uv Python location:

```bash
uv run python -c "import sys; print(sys.executable)"
# Example output: /home/user/.local/share/uv/python/cpython-3.12.12-linux-x86_64-gnu/bin/python3.12
```

2. Set LD_LIBRARY_PATH:

```bash
# For Python 3.12 (adjust version as needed)
export LD_LIBRARY_PATH="$HOME/.local/share/uv/python/cpython-3.12.12-linux-x86_64-gnu/lib:$LD_LIBRARY_PATH"
```

Differential Revision: D86537232

Author: JenniferWang

monarch_extension/Cargo.toml

@@ -34,7 +34,7 @@ monarch_tensor_worker = { version = "0.0.0", path = "../monarch_tensor_worker",
 monarch_types = { version = "0.0.0", path = "../monarch_types" }
 nccl-sys = { path = "../nccl-sys", optional = true }
 ndslice = { version = "0.0.0", path = "../ndslice" }
-pyo3 = { version = "0.24", features = ["anyhow", "multiple-pymethods", "py-clone"] }
+pyo3 = { version = "0.24", features = ["anyhow", "extension-module", "multiple-pymethods", "py-clone"] }
 rdmaxcel-sys = { path = "../rdmaxcel-sys", optional = true }
 serde = { version = "1.0.219", features = ["derive", "rc"] }
 tokio = { version = "1.47.1", features = ["full", "test-util", "tracing"] }

setup.py

@@ -121,22 +121,10 @@ def run(self):
     readme = f.read()
 
 if sys.platform.startswith("linux"):
-    # Always include the active env's lib (Conda-safe)
+    # With extension-module, we don't link libpython, but we still need
+    # RPATH for finding libtorch and other PyTorch libraries
     conda_lib = os.path.join(sys.prefix, "lib")
 
-    # Only use LIBDIR if it actually contains the current libpython
-    ldlib = sysconfig.get_config_var("LDLIBRARY") or ""
-    libdir = sysconfig.get_config_var("LIBDIR") or ""
-    py_lib = ""
-    if libdir and ldlib:
-        cand = os.path.join(libdir, ldlib)
-        if os.path.exists(cand) and os.path.realpath(libdir) != os.path.realpath(
-            conda_lib
-        ):
-            py_lib = libdir
-
-    # Prefer sidecar .so next to the extension; then the conda env;
-    # then (optionally) py_lib
     flags = [
         "-C",
         "link-arg=-Wl,--enable-new-dtags",
@@ -145,14 +133,8 @@ def run(self):
         "-C",
         "link-arg=-Wl,-rpath,$ORIGIN",
         "-C",
-        "link-arg=-Wl,-rpath,$ORIGIN/..",
-        "-C",
-        "link-arg=-Wl,-rpath,$ORIGIN/../../..",
-        "-C",
-        "link-arg=-Wl,-rpath," + conda_lib,
+        "link-arg=-Wl,-rpath," + conda_lib,  # For libtorch
     ]
-    if py_lib:
-        flags += ["-C", "link-arg=-Wl,-rpath," + py_lib]
 
     cur = os.environ.get("RUSTFLAGS", "")
     os.environ["RUSTFLAGS"] = (cur + " " + " ".join(flags)).strip()

torch-sys/Cargo.toml

@@ -19,7 +19,7 @@ hyperactor = { version = "0.0.0", path = "../hyperactor" }
 monarch_types = { version = "0.0.0", path = "../monarch_types" }
 nccl-sys = { path = "../nccl-sys", optional = true }
 paste = "1.0.14"
-pyo3 = { version = "0.24", features = ["anyhow", "multiple-pymethods", "py-clone"] }
+pyo3 = { version = "0.24", features = ["anyhow", "extension-module", "multiple-pymethods", "py-clone"] }
 regex = "1.11.1"
 serde = { version = "1.0.219", features = ["derive", "rc"] }
 thiserror = "2.0.12"