Merge #1218

1218: Require weight and bias to be AbstractArrays r=CarloLucibello a=oxinabox

closes #1199
While in theory someone could be using Dense with weights and biases that are not abstract arrays, I would be surprised.
So allowing it is just leaving a food-gun laying around.
If it is common then we can instead close #1199 by adding a special constructor for `Number` subtypes that error if they are not integers, or something a long those lines.

### PR Checklist

- [x] Tests are added
- [x] Entry in NEWS.md

I think this is a bug-fix thus the following are not required:

- [ ] Documentation, if applicable
- [ ] Final review from `@MikeInnes` or `@dhairyagandhi96` (for API changes).


Co-authored-by: Lyndon White <lyndon.white@invenialabs.co.uk>
Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>

NEWS.md

@@ -1,5 +1,6 @@
 # v0.11
 * Change to `DataLoader`'s constructor [https://github.com/FluxML/Flux.jl/pull/1152] 
+* Error if Dense layers weights and biases are not arrays [https://github.com/FluxML/Flux.jl/pull/1218].
 
 # v0.10.5
 * Add option for [same padding](https://github.com/FluxML/Flux.jl/pull/901) to conv and pooling layers by setting `pad=SamePad()`.

Project.toml

@@ -1,6 +1,6 @@
 name = "Flux"
 uuid = "587475ba-b771-5e3f-ad9e-33799f191a9c"
-version = "0.11.0"
+version = "0.11.0-DEV"
 
 [deps]
 AbstractTrees = "1520ce14-60c1-5f80-bbc7-55ef81b5835c"

src/layers/basic.jl

@@ -102,7 +102,7 @@ julia> d(rand(5))
   -0.16210233
    0.12311903```
 """
-struct Dense{F,S,T}
+struct Dense{F,S<:AbstractArray,T<:AbstractArray}
   W::S
   b::T
   σ::F

test/layers/basic.jl

@@ -28,6 +28,14 @@ import Flux: activations
   end
 
   @testset "Dense" begin
+    @testset "constructors" begin
+      @test size(Dense(10, 100).W) == (100, 10)
+      @test Dense(rand(100,10), rand(10)).σ == identity
+
+      @test_throws MethodError Dense(10, 10.5)
+      @test_throws MethodError Dense(10, 10.5, tanh)
+    end
+
     @test  length(Dense(10, 5)(randn(10))) == 5
     @test_throws DimensionMismatch Dense(10, 5)(randn(1))
     @test_throws MethodError Dense(10, 5)(1) # avoid broadcasting
@@ -37,7 +45,6 @@ import Flux: activations
     @test Dense(10, 1, identity, initW = ones, initb = zeros)(ones(10,2)) == 10*ones(1, 2)
     @test Dense(10, 2, identity, initW = ones, initb = zeros)(ones(10,1)) == 10*ones(2, 1)
     @test Dense(10, 2, identity, initW = ones, initb = zeros)([ones(10,1) 2*ones(10,1)]) == [10 20; 10 20]
-
   end
 
   @testset "Diagonal" begin