[onert] Fix wrong applied shape in backward of MeanLayer

This commit fixes wrong applied shape in backward of MeanLayer.

ONE-DCO-1.0-Signed-off-by: ragmani <ragmani0216@gmail.com>

runtime/onert/backend/train/ops/MeanLayer.cc

@@ -48,26 +48,11 @@ void MeanLayer::forward(bool) { cpu::ops::MeanLayer::run(); }
 
 void MeanLayer::backward()
 {
-  nnfw::cker::Shape keep_dim_shape;
-  if (_keep_dims == false)
-  {
-    keep_dim_shape.ReplaceWith(getShape(_input));
-    auto axes_vec = cpu::ops::getReducerAxes(_axes);
-    for (const auto &axis : axes_vec)
-    {
-      keep_dim_shape.SetDim(axis, 1);
-    }
-  }
-  else
-  {
-    keep_dim_shape.ReplaceWith(getShape(_back_prop_input));
-  }
-
   switch (_back_prop_output->data_type())
   {
     case OperandType::FLOAT32:
     {
-      nnfw::cker::train::MeanGrad(keep_dim_shape, getBuffer<float>(_back_prop_output),
+      nnfw::cker::train::MeanGrad(getShape(_back_prop_output), getBuffer<float>(_back_prop_output),
                                   getShape(_back_prop_input), getBuffer<float>(_back_prop_input));
       break;
     }