Refine merge_selected_rows Doc (#14748)

* add doc for MergeSelectedRows
test=develop

* checkout selected_rows
test=develop

paddle/fluid/operators/merge_selected_rows_op.cc

@@ -26,6 +26,13 @@ class MergeSelectedRowsOp : public framework::OperatorWithKernel {
                    "Input(X) of MergeSelectedRowsOp should not be null.");
     PADDLE_ENFORCE(ctx->HasOutput("Out"),
                    "Output(Out) of MergeSelectedRowsOp should not be null.");
+    PADDLE_ENFORCE_EQ(ctx->GetInputsVarType("X").front(),
+                      framework::proto::VarType::SELECTED_ROWS,
+                      "Input X only should be SelectedRows.");
+    PADDLE_ENFORCE_EQ(ctx->GetOutputsVarType("Out").front(),
+                      framework::proto::VarType::SELECTED_ROWS,
+                      "Output Y only should be SelectedRows.");
+
     ctx->ShareDim("X", /*->*/ "Out");
   }
 };
@@ -43,7 +50,28 @@ class MergeSelectedRowsOpMaker : public framework::OpProtoAndCheckerMaker {
         R"DOC(
 MergeSelectedRows Operator.
 
-MergeSelectedRows is used to merge the duplicated rows of the input.
+MergeSelectedRows is used to merge the duplicated rows of the input. The
+output's row has no duplicated, and it's order is incremental.
+
+Example:
+  Input:
+    X.rows is [0, 5, 5, 4, 19]
+    X.height is 20
+    X.value is:
+        [[1, 1]
+         [2, 2]
+         [3, 3]
+         [4, 4]
+         [6, 6]]
+
+   Output:
+    Out.row is [0, 4, 5, 19]
+    Out.height is 20
+    Out.value is:
+        [[1, 1]
+         [4, 4]
+         [5, 5]
+         [6, 6]]
 )DOC");
   }
 };

python/paddle/fluid/tests/unittests/test_get_tensor_from_selected_rows_op.py

@@ -29,7 +29,7 @@ class TestGetTensorFromSelectedRows(unittest.TestCase):
 
     def check_with_place(self, place):
         scope = core.Scope()
-        x_rows = [0, 5, 5, 4, 20]
+        x_rows = [0, 5, 5, 4, 19]
         height = 20
         row_numel = 2
 

python/paddle/fluid/tests/unittests/test_merge_selectedrows_op.py

@@ -29,8 +29,8 @@ class TestMergeSelectedRows(unittest.TestCase):
 
     def check_with_place(self, place):
         scope = core.Scope()
-        x_rows = [0, 5, 5, 4, 20]
-        out_rows = [0, 4, 5, 20]
+        x_rows = [0, 5, 5, 4, 19]
+        out_rows = [0, 4, 5, 19]
         height = 20
         row_numel = 2