support constant arrays in MXNet

src/backend/mxnet/graph.jl

@@ -73,6 +73,8 @@ end
 
 graph{T<:AArray}(ctx::Context, p::Constant{Flux.Param{T}}) = var(ctx, p.value)
 
+graph{T<:AArray}(ctx::Context, p::Constant{T}) = var(ctx, p.value)
+
 graph(ctx::Context, p::Constant) = p.value
 
 function graph(ctx::Context, model, args...)

src/backend/mxnet/model.jl

@@ -22,9 +22,9 @@ struct Graph
   stacks::Dict{Any,Any}
 end
 
-function mxparams(g::Graph)
+function mxparams(ps)
   params = Dict{Symbol,MXArray}()
-  for (name, param) in g.params
+  for (name, param) in ps
     params[name] = MXArray(size(param))
   end
   return params
@@ -52,8 +52,9 @@ dictt(xs, ys) = Dict(zip(collectt(xs), collectt(ys)))
 
 function executor(graph::Graph, input...)
   shapecheckt(graph.input, input)
-  args  = merge(mxparams(graph), dictt(graph.input, mapt(d->MXArray(size(d)), input)))
-  grads = merge(mxparams(graph), dictt(graph.input, mapt(d->MXArray(size(d)), input)))
+  args  = merge(mxparams(graph.params), dictt(graph.input, mapt(d->MXArray(size(d)), input)))
+  grads = filter((a, b) -> b isa Flux.Param, graph.params)
+  grads = merge(mxparams(grads), dictt(graph.input, mapt(d->MXArray(size(d)), input)))
   exec = mx.bind(mxgroup(graph.output),
                  args = ndparams(args),
                  args_grad = ndparams(grads),
@@ -77,6 +78,7 @@ end
 
 function Flux.update!(exec::Exec, η)
   for (arg, grad) in zip(exec.exec.arg_arrays, exec.exec.grad_arrays)
+    grad == nothing && continue
     mx.@nd_as_jl rw = (arg, grad) begin
       arg .-= grad .* η
       grad[:] = 0
@@ -145,6 +147,6 @@ function mx.FeedForward(model::Flux.Model; input = :data, label = :softmax, cont
   model = rewrite_softmax(model, label)
   graph = tograph(model, input, feedforward=true)
   ff = mx.FeedForward(graph.output, context = context)
-  isempty(graph.params) || (ff.arg_params = ndparams(mxparams(graph)))
+  isempty(graph.params) || (ff.arg_params = ndparams(mxparams(graph.params)))
   return ff
 end