Added comments to lpnormpool

Author: IBArbitrary

src/impl/pooling_direct.jl

@@ -95,14 +95,14 @@ for name in (:max, :mean, :lpnorm)
                 elseif $(name == :mean)
                     m += x[input_kw, input_kh, input_kd, c, batch_idx]
                 elseif $(name == :lpnorm)
-                    # y = (∑ᵢ xᵢ^p)^(1 / p), here to calculate ∑ᵢ xᵢ^p
+                    # y = (∑ᵢ |xᵢ|^p)^(1 / p), here to calculate ∑ᵢ |xᵢ|^p
                     m += abs(x[input_kw, input_kh, input_kd, c, batch_idx])^p
                 else
                     error("Unimplemented codegen path")
                 end
             end
 
-            # for lpnormpool, y = (∑ᵢ xᵢ^p)^(1 / p)
+            # for lpnormpool, y = (∑ᵢ |xᵢ|^p)^(1 / p)
             m = $(name == :lpnorm) ? m^(T(1) / p) : m
 
             y[w, h, d, c, batch_idx] = _alpha * m # + _beta * y[w, h, d, c, batch_idx]
@@ -263,7 +263,7 @@ for name in (:max, :mean, :lpnorm)
                     # Either does meanpool :(
                     dx[input_kw, input_kh, input_kd, c, batch_idx] += dy_idx * _alpha
                 elseif $(name == :lpnorm)
-                    # y = (∑ᵢ xᵢ^p)^(1 / p), ∂y/∂xᵢ = xᵢ^(p-1) × y^(1-p)
+                    # y = (∑ᵢ |xᵢ|^p)^(1 / p), ∂y/∂xᵢ = |xᵢ|^(p-1) × y^(1-p) × sign(xᵢ)
                     xv = x[input_kw, input_kh, input_kd, c, batch_idx]
                     grad = abs(xv)^(p-1) * y_idx^(1-p) * sign(xv)
                     dx[input_kw, input_kh, input_kd, c, batch_idx] += dy_idx * grad