Add sensitivities for float (#143)

`float` acts on arrays and scalars, turning integers into floats of the
corresponding size (e.g. `float(::Int32)` -> `Float32`) and is a no-op
for floats. This same behavior is applied when differentiating.

src/sensitivities/scalar.jl

@@ -44,3 +44,7 @@ end
 
 # Add method to resolve exponentiation ambiguity.
 ^(n::Node{<:Real}, p::Integer) = invoke(^, Tuple{Node{<:Real}, Real}, n, p)
+
+import Base: float
+@explicit_intercepts float Tuple{∇ArrayOrScalar}
+∇(::typeof(float), ::Type{Arg{1}}, p, y, ȳ, x) = float(ȳ)

test/sensitivities/scalar.jl

@@ -91,4 +91,24 @@ end
  # Test whether the exponentiation amibiguity is resolved.
  @test ∇(x -> x^2)(1) == (2.0,)
  end
+
+ @testset "float" begin
+ # Scalars
+ x_ = 4
+ x = Leaf(Tape(), x_)
+ y = float(x)
+ @test y isa Branch{Float64}
+ @test unbox(y) == 4.0
+
+ # Arrays
+ X_ = [1,2,3,4]
+ X = Leaf(Tape(), X_)
+ Y = float(X)
+ @test Y isa Branch{Vector{Float64}}
+ @test unbox(Y) == Float64[1,2,3,4]
+
+ # In expressions
+ @test ∇(x->2x)(1) === (2,)
+ @test ∇(x->2*float(x))(1) === (2.0,)
+ end
 end