vae_mnist: Mutating arrays is not supported

[jstrube]

When running vae_mnist with Flux v0.12.3 causes:

 ERROR: LoadError: Mutating arrays is not supported
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:33
  [2] (::Zygote.var"#411#412")(#unused#::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/lib/array.jl:65
  [3] (::Zygote.var"#2279#back#413"{Zygote.var"#411#412"})(Δ::Nothing)
    @ Zygote ~/.julia/packages/ZygoteRules/OjfTt/src/adjoint.jl:59
  [4] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:45 [inlined]
  [5] (::typeof(∂(params!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
  [6] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:47 [inlined]
  [7] (::typeof(∂(params!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
  [8] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:47 [inlined]
  [9] (::typeof(∂(params!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [10] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:47 [inlined]
 [11] (::typeof(∂(params!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [12] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:44 [inlined]
 [13] (::typeof(∂(params!)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [14] Pullback
    @ ~/.julia/packages/Flux/6o4DQ/src/functor.jl:53 [inlined]
 [15] (::typeof(∂(params)))(Δ::Nothing)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [16] Pullback
    @ /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:66 [inlined]
 [17] (::typeof(∂(model_loss)))(Δ::Float32)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [18] Pullback
    @ /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:142 [inlined]
 [19] (::typeof(∂(λ)))(Δ::Float32)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface2.jl:0
 [20] (::Zygote.var"#69#70"{Zygote.Params, typeof(∂(λ)), Zygote.Context})(Δ::Float32)
    @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface.jl:252
 [21] train(; kws::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Main /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:144
 [22] train()
    @ Main /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:94
 [23] top-level scope
    @ /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:176
in expression starting at /media/jstrube/24988e98-7094-4b00-aeb9-5c98c5b52a901/Data/model-zoo/vision/vae_mnist/vae_mnist.jl:175

According to discourse, this can be fixed by adding Flux.Zygote.@nograd Flux.params before the function definition of model_loss. This indeed fixes it for me.