ndarray: change internal api of plus to help autograd

address #274 (comment)

src/ndarray.jl

@@ -587,8 +587,10 @@ Summation. Multiple arguments of either scalar or `NDArray` could be
 added together. Note at least the first or second argument needs to be an
 `NDArray` to avoid ambiguity of built-in summation.
 """
-+(x::NDArray, ys::NDArrayOrReal...)          = add_to!(copy(x, context(x)), ys...)
-+(x::Real, y::NDArray, zs::NDArrayOrReal...) = add_to!(copy(y, context(y)), x, zs...)
++(x::NDArray)             = x
++(x::NDArray, y::NDArray) = _plus(x, y)
++(x::NDArray, y::Real)    = _plus_scalar(x, scalar = y)
++(y::Real,    x::NDArray) = _plus_scalar(x, scalar = y)
 
 broadcast_(::typeof(+), x::NDArray, y::NDArrayOrReal) = x + y
 broadcast_(::typeof(+), x::Real, y::NDArray)          = x + y
@@ -1176,20 +1178,16 @@ function _get_ndarray_function_def(name :: String)
         args = MX_handle[]
       end
 
-      if length(output_vars) > 0
-        output_handles = map((x) -> Base.cconvert(MX_handle, x), output_vars)
-        # XXX: Julia 0.4 has bug: [Array{MX_handle}] == Array{MX_handle}
-        output_handles_pp = Array{Array{MX_handle}}(1)
-        output_handles_pp[1] = Base.cconvert(Ptr{MX_handle}, output_handles)
+      output_handles_pp = if length(output_vars) > 0
+        [map(x -> x.handle, output_vars)]
       else
-        output_handles_pp = [Base.convert(Ptr{MX_handle}, 0)]
+        [Ptr{MX_handle}(C_NULL)]
       end
       num_outputs_p = [convert(Cint, num_outputs)]
 
       kw_keys_str = String[string(x[1]) for x in kwargs]
       kw_vals_str = String[dump_mx_param(x[2]) for x in kwargs]
 
-      #op_handle = _get_cached_libmx_op_handle($(QuoteNode(name)))
       op_handle = _get_cached_libmx_op_handle($(name))
       @mxcall(:MXImperativeInvoke,
               (MX_handle, Cint, Ptr{MX_handle},