Remove unused this captures in activation lambdas (closes #460)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Dobiasd

include/fdeep/layers/hard_tanh_layer.hpp

@@ -25,7 +25,7 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
-            return transform_tensor([this](float_type x) {
+            return transform_tensor([](float_type x) {
                 if (x < -1) {
                     return static_cast<float_type>(-1);
                 } else if (x > 1) {

include/fdeep/layers/log_sigmoid_layer.hpp

@@ -24,7 +24,7 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
-            return transform_tensor([this](float_type x) {
+            return transform_tensor([](float_type x) {
                 return std::log(1 / (1 + std::exp(-x)));
             },
                 in_vol);

include/fdeep/layers/log_softmax_layer.hpp

@@ -25,7 +25,7 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
-            return transform_tensor([this](float_type x) {
+            return transform_tensor([](float_type x) {
                 return std::log(x);
             },
                 softmax(in_vol));

include/fdeep/layers/sparse_plus_layer.hpp

@@ -24,7 +24,7 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
-            return transform_tensor([this](float_type x) {
+            return transform_tensor([](float_type x) {
                 if (x <= -1) {
                     return static_cast<float_type>(0);
                 } else if (x < 1) {

include/fdeep/layers/tanh_shrink_layer.hpp

@@ -24,7 +24,7 @@ namespace internal {
     protected:
         tensor transform_input(const tensor& in_vol) const override
         {
-            return transform_tensor([this](float_type x) {
+            return transform_tensor([](float_type x) {
                 return x - std::tanh(x);
             },
                 in_vol);