implement mxnet backward pass

src/backend/mxnet/model.jl

@@ -1,6 +1,7 @@
 type MXModel
   model::Any
   params::Dict{Symbol,Any}
+  grads::Dict{Symbol,Any}
   exec::mx.Executor
 end
 
@@ -9,6 +10,15 @@ mxdims(dims::NTuple) =
 
 mxdims(n::Integer) = mxdims((n,))
 
+function tond!(nd::mx.NDArray, xs::AArray)
+  mx.copy_ignore_shape!(nd, xs')
+  nd
+end
+
+tond(xs::AArray) = tond!(mx.zeros(mxdims(size(xs))), xs)
+
+fromnd(xs::mx.NDArray) = copy(xs)'
+
 function mxargs(args)
   map(args) do kv
     arg, value = kv
@@ -16,10 +26,16 @@ function mxargs(args)
   end
 end
 
+function mxgrads(mxargs)
+  map(mxargs) do kv
+    arg, value = kv
+    arg => mx.zeros(size(value))
+  end
+end
+
 function load!(model::MXModel)
   for (name, arr) in model.exec.arg_dict
-    # TODO: don't allocate here
-    haskey(model.params, name) && mx.copy_ignore_shape!(arr, model.params[name]')
+    haskey(model.params, name) && tond!(arr, model.params[name])
   end
   return model
 end
@@ -28,14 +44,24 @@ function mxnet(model::Model, input)
   vars = Dict{Symbol,Any}()
   node = graph(vars, model, mx.Variable(:input))
   args = merge(mxargs(vars), Dict(:input => mx.zeros(mxdims(input))))
-  model = MXModel(model, vars, mx.bind(node, args = args, grad_req = mx.GRAD_NOP))
+  grads = mxgrads(args)
+  model = MXModel(model, vars, grads,
+                  mx.bind(node, args = args,
+                                args_grad = grads,
+                                grad_req = mx.GRAD_ADD))
   load!(model)
   return model
 end
 
 function (model::MXModel)(input)
-  inputnd = model.exec.arg_dict[:input]
-  mx.copy_ignore_shape!(inputnd, input')
+  tond!(model.exec.arg_dict[:input], input)
   mx.forward(model.exec)
-  copy(model.exec.outputs[1])'
+  fromnd(model.exec.outputs[1])
+end
+
+function Flux.back!(model::MXModel, Δ, x)
+  input = model.grads[:input]
+  copy!(input, mx.zeros(size(input)))
+  mx.backward(model.exec, tond(Δ))
+  fromnd(input)
 end