Add test cases for external ray cluster

Author: AnesBenmerzoug

tests/utils/conftest.py

@@ -1,8 +1,25 @@
 import pytest
+import ray
+from ray.cluster_utils import Cluster
 
 from pydvl.utils.config import ParallelConfig
 
 
-@pytest.fixture(scope="session", params=["sequential", "ray"])
+@pytest.fixture(scope="session", params=["sequential", "ray-local", "ray-external"])
 def parallel_config(request):
-    return ParallelConfig(backend=request.param)
+    if request.param == "sequential":
+        yield ParallelConfig(backend=request.param)
+    elif request.param == "ray-local":
+        yield ParallelConfig(backend="ray")
+        ray.shutdown()
+    elif request.param == "ray-external":
+        # Starts a head-node for the cluster.
+        cluster = Cluster(
+            initialize_head=True,
+            head_node_args={
+                "num_cpus": 4,
+            },
+        )
+        yield ParallelConfig(backend="ray", address=cluster.address)
+        ray.shutdown()
+        cluster.shutdown()