[Bugfix] reshape with zero input tensor (#35642)

* reshape support zero-input

* add unitest

* revise error message

paddle/fluid/operators/reshape_op.cc

@@ -185,6 +185,8 @@ class ReshapeOp : public framework::OperatorWithKernel {
                 framework::make_ddim(shape), i, shape[i]));
       }
 
+      // NOTE all non-zero values will be converted to True (include negative
+      // value)
       capacity *= (shape[i] ? shape[i] : in_dims[i]);
       output_shape[i] =
           (shape[i] ? static_cast<int64_t>(shape[i]) : in_dims[i]);
@@ -222,6 +224,21 @@ class ReshapeOp : public framework::OperatorWithKernel {
                 in_dims, in_size, framework::make_ddim(shape), capacity));
       }
     }
+
+    // support reshape with zero-input(input tensor with product(shape) == 0)
+    // by now we require that if the input tensor is zero shape, the target
+    // shape of output must be zero
+    if (in_size == 0) {
+      PADDLE_ENFORCE_EQ(
+          capacity, in_size,
+          platform::errors::InvalidArgument(
+              "The 'shape' in ReshapeOp is invalid. "
+              "The input tensor X's shape = [%s], X's capacity = %d."
+              "But the target shape of Out is [%s],  the "
+              "capacity of 'Out' is %d.",
+              in_dims, in_size, framework::make_ddim(shape), capacity));
+    }
+
     return framework::make_ddim(output_shape);
   }
 

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

@@ -464,5 +464,19 @@ class TestDygraphReshapeInplaceAPI(TestDygraphReshapeAPI):
         self.reshape = paddle.reshape_
 
 
+class TestReshapeZeroTensor(unittest.TestCase):
+    def test_reshape_zero_tensor_success(self):
+        zero_tensor = paddle.zeros([0, 2, 3])
+        # since we use "0" as the dimension copy semantically in reshape, 
+        # we need to copy the 0 dim in the src tensor in order to make a successful zero tensor reshape
+        zero_tensor = zero_tensor.reshape([0, 6])
+        self.assertTrue(list(zero_tensor.shape) == [0, 6])
+
+    def test_reshape_zero_tensor_error(self):
+        zero_tensor = paddle.zeros([0, 2, 3])
+        with self.assertRaises(ValueError):
+            zero_tensor.reshape([2, 3])
+
+
 if __name__ == "__main__":
     unittest.main()