Select[
  trainedNet[{trainSet[[1, 1, 1]]}, NetPortGradient["Input"]] // Flatten,
  # > 0 &] // Histogram

gl[a_] :=
 NetTake[trainedNet, a]
mdt = Table[
   Table[
    gl[a][{testSet[[1, k, 1]]}, NetPortGradient["Input"]] // Flatten //
       Abs // Total,
    {a, 1, 12}],
   {k, 1, 20}];
ListPlot[mdt, Joined -> True, PlotRange -> All,
 FrameLabel -> {"Number of layer", "Total@Abs@Gradient"},
 Frame -> {True, True, False, False}]