add support for 0-element Chain

src/layers/basic.jl

@@ -47,6 +47,9 @@ Calculate the forward results of each layers in Chain `c`
 """
 activations(m, x) = activations(Chain(m), x)
 function activations(c::Chain, x)
+    if length(c) == 0
+        return []
+    end
     rst = [c[1](x), ]
     if length(c) == 1
         return rst

test/layers/basic.jl

@@ -9,6 +9,7 @@ import Flux: activations
         @test_nowarn activations(dummy_model[1:end-1], x)
         @test_nowarn activations(dummy_model, x)
 
+        @test activations(Chain(), x)[1] == []
         @test activations(dummy_model, x)[1] == dummy_model[1](x)
         @test activations(dummy_model, x)[2] == x |> dummy_model[1] |> dummy_model[2]
     end