Add hcat() for tracked arrays

[staticfloat]

No description provided.

[staticfloat]

@MikeInnes Hmmm, I just added a failing test case. It looks like @back() isn't working like I expect here:

  Expression: gradtest(hcat, rand(5), rand(5), rand(5, 2))
  DimensionMismatch("cannot broadcast array to have fewer dimensions")
  Stacktrace:
   [1] check_broadcast_shape(::Tuple{Base.OneTo{Int64}}, ::Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}) at ./broadcast.jl:84
   [2] broadcast_c! at ./broadcast.jl:210 [inlined]
   [3] broadcast! at ./broadcast.jl:206 [inlined]
   [4] accum!(::Array{Float64,1}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:29
   [5] back(::Flux.Tracker.Tracked{Array{Float64,1}}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:32
   [6] back(::TrackedArray{…,Array{Float64,1}}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:43
   [7] macro expansion at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:49 [inlined]
   [8] back(::Base.#hcat, ::Array{Float64,2}, ::TrackedArray{…,Array{Float64,1}}, ::TrackedArray{…,Array{Float64,1}}, ::TrackedArray{…,Array{Float64,2}}) at /home/sabae/.julia/v0.6/Flux/src/tracker/array.jl:127
   [9] back_(::Flux.Tracker.Call{Base.#hcat,Tuple{TrackedArray{…,Array{Float64,1}},TrackedArray{…,Array{Float64,1}},TrackedArray{…,Array{Float64,2}}}}, ::Array{Float64,2}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:25
   [10] back(::Flux.Tracker.Tracked{Array{Float64,2}}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:38
   [11] macro expansion at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:49 [inlined]
   [12] (::Flux.Tracker.##48#50)(::TrackedArray{…,Array{Float64,2}}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/array.jl:325
   [13] foreach(::Function, ::Tuple{TrackedArray{…,Array{Float64,2}}}, ::Tuple{Array{Float64,2}}, ::Vararg{Tuple{Array{Float64,2}},N} where N) at ./abstractarray.jl:1734
   [14] back_(::Flux.Tracker.Call{Flux.Tracker.Broadcasted{Base.#sin,Array{ForwardDiff.Dual{Void,Float64,1},2}},Tuple{TrackedArray{…,Array{Float64,2}}}}, ::Array{Float64,2}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:25r
   [15] back(::Flux.Tracker.Tracked{Array{Float64,2}}, ::Array{Float64,2}) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:38
   [16] back(::Base.#sum, ::Float64, ::TrackedArray{…,Array{Float64,2}}) at /home/sabae/.julia/v0.6/Flux/src/tracker/array.jl:164
   [17] back_(::Flux.Tracker.Call{Base.#sum,Tuple{TrackedArray{…,Array{Float64,2}}}}, ::Float64, ::Float64) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:25
   [18] back(::Flux.Tracker.Tracked{Float64}, ::Int64) at /home/sabae/.julia/v0.6/Flux/src/tracker/back.jl:36
   [19] back!(::Flux.Tracker.TrackedReal{Float64}) at /home/sabae/.julia/v0.6/Flux/src/tracker/scalar.jl:11
   [20] gradient(::Function, ::Array{Float64,1}, ::Vararg{AbstractArray,N} where N) at /home/sabae/.julia/v0.6/Flux/src/tracker/numeric.jl:3
   [21] gradcheck(::Function, ::Array{Float64,1}, ::Vararg{Any,N} where N) at /home/sabae/.julia/v0.6/Flux/src/tracker/numeric.jl:22
   [22] macro expansion at /home/sabae/.julia/v0.6/Flux/test/tracker.jl:33 [inlined]
   [23] macro expansion at ./test.jl:860 [inlined]
   [24] anonymous at ./<missing>:?

[staticfloat]

@MikeInnes I fixed this; just needed some index fiddling to get x .+= Δ to work when the operation had expanded the number of dimensions of Δ.

[MikeInnes]

Merged with #243 -- thanks 👍