add doc for ReduceOp (#28051)

* add doc, test=document_fix

python/paddle/distributed/collective.py

@@ -36,7 +36,37 @@
 
 
 class ReduceOp:
-    """Reduce Operation"""
+    """
+    Specify the type of operation used for element-wise reductions.
+    It should be one of the following values:
+
+        ReduceOp.SUM
+
+        ReduceOp.MAX
+
+        ReduceOp.MIN
+
+        ReduceOp.PROD
+
+    Examples:
+        .. code-block:: python
+
+            import numpy as np
+            import paddle
+            from paddle.distributed import ReduceOp
+            from paddle.distributed import init_parallel_env
+
+            paddle.set_device('gpu:%d'%paddle.distributed.ParallelEnv().dev_id)
+            init_parallel_env()
+            if paddle.distributed.ParallelEnv().local_rank == 0:
+                np_data = np.array([[4, 5, 6], [4, 5, 6]])
+            else:
+                np_data = np.array([[1, 2, 3], [1, 2, 3]])
+            data = paddle.to_tensor(np_data)
+            paddle.distributed.all_reduce(data, op=ReduceOp.SUM)
+            out = data.numpy()
+            # [[5, 7, 9], [5, 7, 9]]
+    """
     SUM = 0
     MAX = 1
     MIN = 2