[OpenVINO backend] fix ops.sum (#21463)

Author: Mohamed-Ashraf273

keras/src/backend/openvino/numpy.py

@@ -1654,6 +1654,10 @@ def var(x, axis=None, keepdims=False):
 
 def sum(x, axis=None, keepdims=False):
     x = get_ov_output(x)
+    if axis is None:
+        flatten_shape = ov_opset.constant([-1], Type.i32).output(0)
+        x = ov_opset.reshape(x, flatten_shape, False).output(0)
+        axis = 0
     axis = ov_opset.constant(axis, Type.i32).output(0)
     return OpenVINOKerasTensor(ov_opset.reduce_sum(x, axis, keepdims).output(0))