ensure ids in lookup table op must be a column vector (#4987)

* ensure ids in lookup table op must be a column vector

* follow comments

paddle/operators/lookup_table_op.cc

@@ -32,6 +32,9 @@ class LookupTableOp : public framework::OperatorWithKernel {
     auto table_dims = ctx->GetInputDim("W");
     auto ids_dims = ctx->GetInputDim("Ids");
 
+    PADDLE_ENFORCE_EQ(ids_dims.size(), 2);
+    PADDLE_ENFORCE_EQ(ids_dims[1], 1);
+
     ctx->SetOutputDim("Out", {ids_dims[0], table_dims[1]});
     ctx->ShareLoD("Ids", /*->*/ "Out");
   }
@@ -53,7 +56,9 @@ class LookupTableOpMaker : public framework::OpProtoAndCheckerMaker {
              " which is a learnable parameter.");
     AddInput("Ids",
              "An input with type int32 or int64"
-             "contains the ids to be looked up in W.");
+             "contains the ids to be looked up in W."
+             "Ids must be a column vector with rank = 2."
+             "The 2nd dimension size must be 1");
     AddOutput("Out", "The lookup results, which have the same type with W.");
     AddComment(R"DOC(
 This operator is used to perform lookups on the parameter W,

python/paddle/v2/framework/tests/test_lookup_table_op.py

@@ -8,7 +8,8 @@ class TestLookupTableOp(OpTest):
         self.op_type = "lookup_table"
         table = np.random.random((17, 31)).astype("float32")
         ids = np.random.randint(0, 17, 4).astype("int32")
-        self.inputs = {'W': table, 'Ids': ids}
+        ids_expand = np.expand_dims(ids, axis=1)
+        self.inputs = {'W': table, 'Ids': ids_expand}
         self.outputs = {'Out': table[ids]}
 
     def test_check_output(self):