Inspecting intermediate values in a neural network

You can also tap off the intermediate layers by attaching output ports. For example

net = NetInitialize@
  NetGraph[{LongShortTermMemoryLayer[5], SequenceLastLayer[], 
    LinearLayer[2], 
    SoftmaxLayer[]}, {NetPort["Input"] -> 
     1 -> 2 -> 3 -> 4 -> NetPort["Output"], 1 -> NetPort["layer1"], 
    2 -> NetPort["layer2"], 3 -> NetPort["layer3"]}, 
   "Input" -> {"Varying", 3}]

Then

net[{{1, 2, 3}, {0, 1, 3}}]
(* <|"Output" -> {0.716093, 0.283907}, 
 "layer1" -> {{0.0242801, 0.00128676, -0.220564, 0.010784, 
    0.369972}, {-0.0774157, 0.0137297, -0.456915, 0.0116218, 
    0.665373}}, 
 "layer2" -> {-0.0774157, 0.0137297, -0.456915, 0.0116218, 0.665373}, 
 "layer3" -> {0.748411, -0.176754}|> *)

You don't have to use all the output ports in the training process. For example, this uses only the original input and output ports:

NetTrain[net, {<|"Input" -> {{1, 2, 3}, {0, 1, 3}}, 
   "Output" -> {0.5281522274017334`, 0.471847802400589`}|>},
   "Output" -> MeanSquaredLossLayer[]]

This works for NetGraph as well.

---

Here is a complete example using MNIST where the 10th layer is tapped off:

lenet = NetGraph[{
   ConvolutionLayer[20, 5], Ramp, PoolingLayer[2, 2],
   ConvolutionLayer[50, 5], Ramp, PoolingLayer[2, 2],
   FlattenLayer[], 500, Ramp, 10, 
   SoftmaxLayer[]}, {NetPort["Input"] -> 
    1 -> 2 -> 
      3 -> 4 -> 
        5 -> 6 -> 7 -> 8 -> 9 -> 10 -> 11 -> NetPort["Output"], 
   10 -> NetPort["layer10"]},
  "Input" -> NetEncoder[{"Image", {28, 28}, "Grayscale"}], 
  "Output" -> NetDecoder[{"Class", Range[0, 9]}]
  ]

trainingData2 = <|"Input" -> #[[1]], "Output" -> #[[2]]|> & /@ 
   RandomSample[trainingData, 1000];

trained = NetTrain[lenet, trainingData2, "Output" -> CrossEntropyLossLayer["Index"]]

trained[trainingData2[[1, 1]]]
(* <|"Output" -> 5, 
 "layer10" -> {-2.34726, -9.10603, -0.501655, -0.602975, 0.368083, 
   6.48835, -5.21255, -1.2072, 1.68743, 5.18232}|> *)

Not really "convenient", but you can get the layers of a NetChain using Normal:

net = NetInitialize@NetChain[{3, Ramp, 4, Ramp, 1}, "Input" -> "Real"];
layers = Normal[net]

and then construct a NetGraph with the same layers, and an output port for each layer:

g = NetGraph[layers,
  Join[
   (* connections between adjacent layers *)
   # -> # + 1 & /@ Range[Length[layers] - 1],
   (* output ports *)
   # -> NetPort[StringTemplate["Layer_``"][#]] & /@ 
    Range[Length[layers]]
   ]]

Result:

net[1]

{0.168289}

g[1]

<|"Layer_1" -> {-0.792354, 0.213135, 0.565724}, "Layer_2" -> {0., 0.213135, 0.565724}, "Layer_3" -> {-0.422488, 0.126484, 0.000964101, 0.0990122}, "Layer_4" -> {0., 0.126484, 0.000964101, 0.0990122}, "Layer_5" -> {0.168289}|>