chore: import upstream snapshot with attribution

python/ray/train/collective/__init__.py

@@ -0,0 +1,19 @@
+from ray.train.v2._internal.constants import is_v2_enabled
+
+if is_v2_enabled():
+    from ray.train.collective.collectives import barrier, broadcast_from_rank_zero
+
+    __all__ = [
+        "broadcast_from_rank_zero",
+        "barrier",
+    ]
+
+    broadcast_from_rank_zero.__module__ = "ray.train.collective"
+    barrier.__module__ = "ray.train.collective"
+else:
+    raise ImportError(
+        "`ray.train.collective` is only available in Ray Train v2. "
+        "To enable it, please set `RAY_TRAIN_V2_ENABLED=1`."
+    )
+
+# DO NOT ADD ANYTHING AFTER THIS LINE.