【Zero-Dim】Flatten support 0d tensor (#49361)

* flatten op support 0D-tensor

* add test in zero dim py

* fix shape should be list

* short code for ci-coverage

* add backward test

* simple code for ci coverage

* add axis check

* add 0D-tensor test in test_flatten_contiguous_range_op.py

* add axis error test for Coverage CI

* add more test for CI-Coverage

* add more test for CI-Coverage

paddle/fluid/operators/flatten_op.cc

@@ -41,11 +41,13 @@ class FlattenOp : public framework::OperatorWithKernel {
                       0,
                       platform::errors::InvalidArgument(
                           "The axis should be greater than or equal to 0."));
+    if (in_dims.size() > 0) {
       PADDLE_ENFORCE_LE(
           axis,
           in_dims.size(),
           platform::errors::InvalidArgument(
               "The axis should be less than or equal to input tensor's rank."));
+    }
 
     const auto &out_dims = GetOutputShape(axis, in_dims);
     ctx->SetOutputDim("Out", phi::make_ddim(out_dims));
@@ -58,6 +60,10 @@ class FlattenOp : public framework::OperatorWithKernel {
 
   static std::vector<int32_t> GetOutputShape(const int axis,
                                              const framework::DDim &in_dims) {
+    if (in_dims.size() == 0) {
+      return {1};
+    }
+
     int64_t outer = 1, inner = 1;
     for (int i = 0; i < in_dims.size(); ++i) {
       if (i < axis) {

paddle/fluid/operators/flatten_op.h

@@ -49,6 +49,10 @@ class FlattenKernel : public framework::OpKernel<T> {
 
   static std::vector<int32_t> GetOutputShape(const int axis,
                                              const framework::DDim &in_dims) {
+    if (in_dims.size() == 0) {
+      return {1};
+    }
+
     int64_t outer = 1, inner = 1;
     for (int i = 0; i < in_dims.size(); ++i) {
       if (i < axis) {

paddle/phi/infermeta/unary.cc

@@ -1097,21 +1097,40 @@ void FlattenWithXShapeInferMeta(const MetaTensor& x,
                                 MetaTensor* xshape) {
   auto x_dims = x.dims();
   int in_dims_size = x_dims.size();
+
+  if (in_dims_size == 0) {
+    PADDLE_ENFORCE_EQ(
+        start_axis == 0 || start_axis == -1,
+        true,
+        phi::errors::InvalidArgument("The start_axis should be 0 or -1 when "
+                                     "the input tensor is a 0D-Tensor"));
+    PADDLE_ENFORCE_EQ(
+        stop_axis == 0 || stop_axis == -1,
+        true,
+        phi::errors::InvalidArgument("The stop_axis should be 0 or -1 when the "
+                                     "input tensor is a 0D-Tensor"));
+    // this can ensure out shape {1}
+    start_axis = 0;
+    stop_axis = -1;
+  }
+
   if (start_axis < 0) {
     start_axis = start_axis + in_dims_size;
   }
   if (stop_axis < 0) {
     stop_axis = stop_axis + in_dims_size;
   }
+  if (in_dims_size > 0) {
     PADDLE_ENFORCE_GE(
         stop_axis,
         start_axis,
         phi::errors::InvalidArgument("The stop_axis should be greater"
                                      "than or equal to start_axis."));
+  }
 
   int64_t outer = 1;
   std::vector<int32_t> out_shape;
-  out_shape.reserve(in_dims_size - stop_axis + start_axis);
+  out_shape.reserve(in_dims_size - stop_axis + start_axis + 1);
 
   for (int i = 0; i < start_axis; ++i) {
     out_shape.push_back(x_dims[i]);

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

@@ -124,6 +124,20 @@ class TestFlattenOp_5(TestFlattenOp):
         }
 
 
+class TestFlattenOp_6(TestFlattenOp):
+    def init_test_case(self):
+        self.in_shape = tuple()
+        self.start_axis = 0
+        self.stop_axis = -1
+        self.new_shape = (1,)
+
+    def init_attrs(self):
+        self.attrs = {
+            "start_axis": self.start_axis,
+            "stop_axis": self.stop_axis,
+        }
+
+
 class TestFlattenOpSixDims(TestFlattenOp):
     def init_test_case(self):
         self.in_shape = (3, 2, 3, 2, 4, 4)
@@ -156,7 +170,7 @@ class TestFlatten2OpError(unittest.TestCase):
             x_var = paddle.static.data(
                 name="x", shape=image_shape, dtype='float32'
             )
-            out = paddle.flatten(x_var, start_axis=2, stop_axis=1)
+            out = paddle.flatten(x_var, start_axis=3, stop_axis=1)
 
         self.assertRaises(ValueError, test_ValueError1)
 
@@ -176,6 +190,22 @@ class TestFlatten2OpError(unittest.TestCase):
 
         self.assertRaises(ValueError, test_ValueError3)
 
+        def test_ValueError4():
+            x_var = paddle.static.data(
+                name="x", shape=image_shape, dtype='float32'
+            )
+            paddle.flatten(x_var, start_axis=2.0, stop_axis=10)
+
+        self.assertRaises(ValueError, test_ValueError4)
+
+        def test_ValueError5():
+            x_var = paddle.static.data(
+                name="x", shape=image_shape, dtype='float32'
+            )
+            paddle.flatten(x_var, start_axis=2, stop_axis=10.0)
+
+        self.assertRaises(ValueError, test_ValueError5)
+
         def test_type():
             # dtype must be float32, float64, int8, int32, int64, uint8.
             x2 = (
@@ -295,5 +325,27 @@ class TestDygraphInplaceFlattenPython(unittest.TestCase):
         paddle.enable_static()
 
 
+class TestFlatten0DTensorOpError(unittest.TestCase):
+    def test_errors(self):
+        image_shape = tuple()
+        x = np.random.uniform(-1.0, 1.0, []).astype('float32')
+
+        def test_ValueError1():
+            x_var = paddle.static.data(
+                name="x", shape=image_shape, dtype='float32'
+            )
+            out = paddle.flatten(x_var, start_axis=10, stop_axis=0)
+
+        self.assertRaises(ValueError, test_ValueError1)
+
+        def test_ValueError2():
+            x_var = paddle.static.data(
+                name="x", shape=image_shape, dtype='float32'
+            )
+            out = paddle.flatten(x_var, start_axis=0, stop_axis=10)
+
+        self.assertRaises(ValueError, test_ValueError2)
+
+
 if __name__ == "__main__":
     unittest.main()

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

@@ -721,6 +721,19 @@ class TestSundryAPI(unittest.TestCase):
         self.assertEqual(out.numpy()[3], 2)
         self.assertEqual(out.grad.shape, [5])
 
+    def test_flatten(self):
+        x = paddle.rand([])
+        x.stop_gradient = False
+
+        start_axis = 0
+        stop_axis = -1
+
+        out = paddle.flatten(x, start_axis=start_axis, stop_axis=stop_axis)
+        out.backward()
+
+        self.assertEqual(out.shape, [1])
+        self.assertEqual(x.grad.shape, [])
+
     def test_scale(self):
         x = paddle.rand([])
         x.stop_gradient = False
@@ -1113,6 +1126,22 @@ class TestSundryAPIStatic(unittest.TestCase):
         self.assertEqual(res[0].shape, (5,))
         self.assertEqual(res[0][3], 2)
 
+    @prog_scope()
+    def test_flatten(self):
+        x = paddle.full([], 1, 'float32')
+        x.stop_gradient = False
+
+        start_axis = 0
+        stop_axis = -1
+
+        out = paddle.flatten(x, start_axis=start_axis, stop_axis=stop_axis)
+        paddle.static.append_backward(out)
+
+        prog = paddle.static.default_main_program()
+        res = self.exe.run(prog, feed={}, fetch_list=[out])
+
+        self.assertEqual(res[0].shape, (1,))
+
     @prog_scope()
     def test_scale(self):
         x = paddle.rand([])

python/paddle/fluid/tests/unittests/xpu/test_zero_dim_tensor_xpu.py

@@ -521,6 +521,19 @@ class TestSundryAPI(unittest.TestCase):
         for i in range(3):
             self.assertEqual(out.numpy()[1][i], updates.numpy()[i])
 
+    def test_flatten(self):
+        x = paddle.full([], 1, 'float32')
+        x.stop_gradient = False
+
+        start_axis = 0
+        stop_axis = -1
+
+        out = paddle.flatten(x, start_axis=start_axis, stop_axis=stop_axis)
+        out.backward()
+
+        self.assertEqual(out.shape, [1])
+        self.assertEqual(x.grad.shape, [])
+
     def test_scale(self):
         x = paddle.rand([])
         x.stop_gradient = False

python/paddle/tensor/manipulation.py

@@ -1550,6 +1550,16 @@ def flatten(x, start_axis=0, stop_axis=-1, name=None):
         )
 
     x_dim = len(x.shape)
+    if x_dim == 0:
+        if not (isinstance(start_axis, int)) or start_axis not in [0, -1]:
+            raise ValueError(
+                "The start_axis should be int, and should be 0 or -1 when the input tensor is a 0D-Tensor"
+            )
+        if not (isinstance(stop_axis, int)) or stop_axis not in [0, -1]:
+            raise ValueError(
+                "The stop_axis should be int, and should be 0 or -1 when the input tensor is a 0D-Tensor"
+            )
+    else:
         if (
             not (isinstance(start_axis, int))
             or (start_axis > x_dim - 1)