Added softmax file in onnx directory

src/onnx/softmax.h

+#include <vector>
+#include <algorithm>
+#include <cmath>
+
+template <typename T>
+std::vector<T> softmax(const std::vector<T>& p)
+{
+    size_t n = p.size();
+    std::vector<T> result(n);
+    std::transform(p.begin(), p.end(), result.begin(), [](auto x) { return std::exp(x); });
+    T s = std::accumulate(result.begin(), result.end(), 0.0f, std::plus<T>());
+    std::transform(result.begin(), result.end(), result.begin(), [=](auto x) { return x / s; });
+    return result;
+}