add bp function for softmax layer

1 files changed, 6 insertions, 1 deletions

diff --git a/nerv/layer/softmax.lua b/nerv/layer/softmax.lua
index f7a5163..81dcacc 100644
--- a/nerv/layer/softmax.lua
+++ b/nerv/layer/softmax.lua
@@ -28,7 +28,12 @@ function SoftmaxLayer:propagate(input, output)
 end
 
 function SoftmaxLayer:back_propagate(bp_err, next_bp_err, input, output)
-    nerv.error_method_not_implemented()
+    local nbe = next_bp_err[1]
+    nbe:mul_elem(bp_err[1], output[1])
+    local offset = nbe:rowsum()
+    nbe:copy_from(bp_err[1])
+    nbe:add_row(offset, -1.0)
+    nbe:mul_elem(nbe, output[1])
 end
 
 function SoftmaxLayer:get_params()