Fix nthreads on 1.9 (failure in the presence of interactive threads)

[IanButterworth]

Currently NNlib threaded functions fail if julia is sstarted with > 0 threads in the :interactive threadpool.

Here julia had -t2,1

BoundsError: attempt to access 196608×9×2 Array{Float32, 3} at index [1:196608, 1:9, 3]
        Stacktrace:
         [1] throw_boundserror(A::Array{Float32, 3}, I::Tuple{Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Int64})
           @ Base ./abstractarray.jl:744
         [2] checkbounds
           @ ./abstractarray.jl:709 [inlined]
         [3] view
           @ ./subarray.jl:177 [inlined]
         [4] macro expansion
           @ ~/.julia/packages/NNlib/Jmwx0/src/impl/conv_im2col.jl:49 [inlined]
         [5] (::NNlib.var"#1110#threadsfor_fun#635"{NNlib.var"#1110#threadsfor_fun#634#636"{Array{Float32, 3}, Float32, Float32, SubArray{Float32, 5, Array{Float32, 5}, Tuple{Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, UnitRange{Int64}, Base.Slice{Base.OneTo{Int64}}}, false}, SubArray{Float32, 5, Array{Float32, 5}, Tuple{Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, UnitRange{Int64}, Base.Slice{Base.OneTo{Int64}}}, false}, SubArray{Float32, 5, Array{Float32, 5}, Tuple{Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, UnitRange{Int64}}, true}, NNlib.DenseConvDims{3, 3, 3, 6, 3}, Int64, Int64, Int64, UnitRange{Int64}}})(tid::Int64; onethread::Bool)

[ToucheSir]

Thanks! Do you know if any NNlib code also suffers from the threadid cache + non-sticky task issue?

[IanButterworth]

I have no idea. Worth checking

[IanButterworth]

Just for the future, it's a little painful that this wasn't merged and released as a patch before #492 given that prompted 0.9.0

[CarloLucibello]

yes, I realized that 30 seconds too late, sorry. I will make a backport in a few hours