Deduplicate and generalise metric update!

MXNet allows for the design of networks that use the same label
for multiple outputs. Instead of failing for these kinds of
networks, warn the user and try to proceed.

src/metric.jl

@@ -36,6 +36,19 @@ set.
 =#
 abstract AbstractEvalMetric
 
+# Generic update! version
+function update!{T <: AbstractEvalMetric}(metric :: T, labels :: Vector{NDArray}, preds :: Vector{NDArray})
+  if length(labels) != length(preds)
+    Base.warn_once(
+      "The number of labels ($(length(labels))) does not correspond to the\
+      number of outputs ($(length(preds))). The calculated metric might not be accuracte.")
+  end
+  for (label, pred) in zip(labels, preds)
+    _update_single_output(metric, label, pred)
+  end
+end
+
+
 #=doc
 .. class:: Accuracy
 
@@ -85,13 +98,6 @@ function _update_single_output(metric :: Accuracy, label :: NDArray, pred :: NDA
   end
 end
 
-function update!(metric :: Accuracy, labels :: Vector{NDArray}, preds :: Vector{NDArray})
-  @assert length(labels) == length(preds)
-  for i = 1:length(labels)
-    _update_single_output(metric, labels[i], preds[i])
-  end
-end
-
 import Base: get
 function get(metric :: Accuracy)
   return [(:accuracy, metric.acc_sum / metric.n_sample)]
@@ -129,13 +135,6 @@ function _update_single_output(metric :: MSE, label :: NDArray, pred :: NDArray)
   end
 end
 
-function update!(metric :: MSE, labels :: Vector{NDArray}, preds :: Vector{NDArray})
-  @assert length(labels) == length(preds)
-  for i = 1:length(labels)
-    _update_single_output(metric, labels[i], preds[i])
-  end
-end
-
 function get(metric :: MSE)
   return [(:MSE, metric.mse_sum / metric.n_sample)]
 end
@@ -193,13 +192,6 @@ function _update_single_output(metric :: ACE, label :: NDArray, pred :: NDArray)
   end
 end
 
-function update!(metric :: ACE, labels :: Vector{NDArray}, preds :: Vector{NDArray})
-  @assert length(labels) == length(preds)
-  for i = 1:length(labels)
-    _update_single_output(metric, labels[i], preds[i])
-  end
-end
-
 #=doc
 .. class:: MultiACE
 
@@ -251,9 +243,3 @@ function _update_single_output(metric :: MultiACE, label :: NDArray, pred :: NDA
   end
 end
 
-function update!(metric :: MultiACE, labels :: Vector{NDArray}, preds :: Vector{NDArray})
-  @assert length(labels) == length(preds)
-  for i = 1:length(labels)
-    _update_single_output(metric, labels[i], preds[i])
-  end
-end