chore: import upstream snapshot with attribution

tools/train/source/optimizer/LearningRateScheduler.cpp

@@ -0,0 +1,46 @@
+//
+//  LearningRateScheduler.cpp
+//  MNN
+//
+//  Created by MNN on 2019/12/05.
+//  Copyright © 2018, Alibaba Group Holding Limited
+//
+
+#include "LearningRateScheduler.hpp"
+#include <algorithm>
+#include <cmath>
+
+namespace MNN {
+namespace Train {
+
+float LrScheduler::multiStep(const float baseLr, const int step, std::vector<int> stepIterations,
+                             std::vector<float> lrMulti) {
+    float lr = baseLr;
+    std::sort(stepIterations.begin(), stepIterations.end());
+    for (int i = 0; i < stepIterations.size(); i++) {
+        if (step == stepIterations[i]) {
+            float multi;
+            if ((i + 1) > lrMulti.size()) {
+                multi = lrMulti[lrMulti.size() - 1];
+            } else {
+                multi = lrMulti[i];
+            }
+            lr *= multi;
+        }
+    }
+
+    return lr;
+}
+
+float LrScheduler::inv(const float baseLr, const int step, const float gamma, const float power) {
+    float lr = baseLr * std::pow(1 + gamma * step, -power);
+    return lr;
+}
+
+float LrScheduler::exp(const float baseLr, const int step, const float gamma) {
+    float lr = baseLr * std::pow(gamma, step);
+    return lr;
+}
+
+} // namespace Train
+} // namespace MNN