cherry pick concat op support negative axis (#18050)

test=release/1.5

paddle/fluid/operators/concat_op.cc

@@ -36,7 +36,10 @@ class ConcatOp : public framework::OperatorWithKernel {
                    "Output(Out) of ConcatOp should not be null.");
 
     auto ins = ctx->GetInputsDim("X");
-    size_t axis = static_cast<size_t>(ctx->Attrs().Get<int>("axis"));
+    size_t axis =
+        ComputeAxis(static_cast<int64_t>(ctx->Attrs().Get<int>("axis")),
+                    static_cast<int64_t>(ins[0].size()));
+
     const size_t n = ins.size();
 
     PADDLE_ENFORCE_GT(n, 0, "Input tensors count should > 0.");
@@ -115,7 +118,10 @@ class ConcatOpMaker : public framework::OpProtoAndCheckerMaker {
         "(bool, default false) Indicates if MKL-DNN kernel will be used")
         .SetDefault(false);
     AddAttr<int>("axis",
-                 "The axis along which the input tensors will be concatenated.")
+                 "The axis along which the input tensors will be concatenated."
+                 "The axis could also be negative numbers. Negative axis is "
+                 "interpreted as counting from the end of the rank."
+                 "i.e., axis + rank(X) th dimension.")
         .SetDefault(0);
     AddAttr<bool>("use_quantizer",
                   "(bool, default false) "

paddle/fluid/operators/concat_op.h

@@ -23,13 +23,22 @@ limitations under the License. */
 namespace paddle {
 namespace operators {
 
+static inline int64_t ComputeAxis(int64_t axis, int64_t rank) {
+  if (axis < 0) {
+    axis = axis + rank;
+  }
+  return axis > 0 ? axis : 0;
+}
+
 template <typename DeviceContext, typename T>
 class ConcatKernel : public framework::OpKernel<T> {
  public:
   void Compute(const framework::ExecutionContext& ctx) const override {
     auto ins = ctx.MultiInput<framework::Tensor>("X");
     framework::Tensor* out = ctx.Output<framework::Tensor>("Out");
-    int64_t axis = static_cast<int64_t>(ctx.Attr<int>("axis"));
+    PADDLE_ENFORCE(ins[0], "The input should not be null.");
+    auto axis = ComputeAxis(static_cast<int64_t>(ctx.Attr<int>("axis")),
+                            static_cast<int64_t>(ins[0]->dims().size()));
     auto place = ctx.GetPlace();
     out->mutable_data<T>(place);
 
@@ -83,8 +92,9 @@ class ConcatGradKernel : public framework::OpKernel<T> {
         }
       }
     }
-
-    int64_t axis = static_cast<int64_t>(ctx.Attr<int>("axis"));
+    PADDLE_ENFORCE(ins[0], "The input should not be null.");
+    auto axis = ComputeAxis(static_cast<int64_t>(ctx.Attr<int>("axis")),
+                            static_cast<int64_t>(ins[0]->dims().size()));
 
     // get output tensor that the name is not kEmptyVarName
     std::vector<framework::Tensor*> outputs;

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

@@ -25,9 +25,15 @@ class TestConcatOp(OpTest):
         self.init_test_data()
         self.inputs = {'X': [('x0', self.x0), ('x1', self.x1), ('x2', self.x2)]}
         self.attrs = {'axis': self.axis}
+        if self.axis < 0:
+            self.actual_axis = self.axis + len(self.x0.shape)
+            self.actual_axis = self.actual_axis if self.actual_axis > 0 else 0
+        else:
+            self.actual_axis = self.axis
+
         self.outputs = {
             'Out': np.concatenate(
-                (self.x0, self.x1, self.x2), axis=self.axis)
+                (self.x0, self.x1, self.x2), axis=self.actual_axis)
         }
 
     def test_check_output(self):
@@ -75,5 +81,13 @@ class TestConcatOp4(TestConcatOp):
         pass
 
 
+class TestConcatOp5(TestConcatOp):
+    def init_test_data(self):
+        self.x0 = np.random.random((2, 1, 4, 5)).astype('float32')
+        self.x1 = np.random.random((2, 2, 4, 5)).astype('float32')
+        self.x2 = np.random.random((2, 3, 4, 5)).astype('float32')
+        self.axis = -3
+
+
 if __name__ == '__main__':
     unittest.main()