fix promotion by avoiding integer division in mse and crossentropy

oops

add tests

src/layers/stateless.jl

@@ -2,16 +2,16 @@ using NNlib: logsoftmax, logσ
 
 # Cost functions
 
-mse(ŷ, y) = sum((ŷ .- y).^2)/length(y)
+mse(ŷ, y) = sum((ŷ .- y).^2) * 1 // length(y)
 
 function crossentropy(ŷ::AbstractVecOrMat, y::AbstractVecOrMat; weight = 1)
-  -sum(y .* log.(ŷ) .* weight) / size(y, 2)
+  -sum(y .* log.(ŷ) .* weight) * 1 // size(y, 2)
 end
 
 @deprecate logloss(x, y) crossentropy(x, y)
 
 function logitcrossentropy(logŷ::AbstractVecOrMat, y::AbstractVecOrMat; weight = 1)
-  return -sum(y .* logsoftmax(logŷ) .* weight) / size(y, 2)
+  return -sum(y .* logsoftmax(logŷ) .* weight) * 1 // size(y, 2)
 end
 
 """

test/layers/stateless.jl

@@ -49,4 +49,16 @@ const ϵ = 1e-7
   @testset "logitbinarycrossentropy" begin
     @test logitbinarycrossentropy.(logŷ, y) ≈ binarycrossentropy.(σ.(logŷ), y; ϵ=0)
   end
+
+  @testset "no spurious promotions" begin
+    for T in (Float16, Float32, Float64)
+      y = rand(T, 2)
+      ŷ = rand(T, 2)
+      for f in (mse, crossentropy, logitcrossentropy)
+        fwd, back = Flux.Tracker.forward(mse, ŷ, y)
+        @test typeof(fwd) == Flux.Tracker.TrackedReal{T}
+        @test eltype(back(one(T))[1]) == Flux.Tracker.TrackedReal{T}
+      end
+    end
+  end
 end