fix maxpool gradient

Author: yiyuezhuo

src/impl/pooling_direct.jl

@@ -176,15 +176,15 @@ for name in (:max, :mean)
                     # If it's equal; this is the one we chose. We only choose one per
                     # kernel window, all other elements of dx must be zero.
                     if y_idx == x[x_idxs...] && !maxpool_already_chose
-                        dx[x_idxs...] = dy_idx*alpha + beta*dx[x_idxs...]
+                        dx[x_idxs...] += dy_idx*alpha + beta*dx[x_idxs...]
                         maxpool_already_chose = true
                     # Maxpooling does not support `beta` right now.  :(
                     #else
                     #    dx[x_idxs...] = T(0) + beta*dx[x_idxs...]
                     end
                 elseif $(name == :mean)
                     # Either does meanpool :(
-                    dx[x_idxs...] = dy_idx*alpha + dx[x_idxs...]
+                    dx[x_idxs...] += dy_idx*alpha + dx[x_idxs...]
                 else
                     error("Unimplemented codegen path")
                 end
@@ -228,7 +228,7 @@ for name in (:max, :mean)
                             x_idxs = (input_kw, input_kh, input_kd, c, batch_idx)
                             if $(name == :max)
                                 if y_idx == x[x_idxs...] && !maxpool_already_chose
-                                    dx[x_idxs...] = dy_idx*alpha + beta*dx[x_idxs...]
+                                    dx[x_idxs...] += dy_idx*alpha + beta*dx[x_idxs...]
                                     maxpool_already_chose = true
                                 #else
                                 #    dx[x_idxs...] = T(0) + beta*dx[x_idxs...]