first commit

ssd/solver/lr_scheduler.py

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+from bisect import bisect_right
+
+from torch.optim.lr_scheduler import _LRScheduler
+
+
+class WarmupMultiStepLR(_LRScheduler):
+    def __init__(self, optimizer, milestones, gamma=0.1, warmup_factor=1.0 / 3,
+                 warmup_iters=500, last_epoch=-1):
+        if not list(milestones) == sorted(milestones):
+            raise ValueError(
+                "Milestones should be a list of" " increasing integers. Got {}",
+                milestones,
+            )
+
+        self.milestones = milestones
+        self.gamma = gamma
+        self.warmup_factor = warmup_factor
+        self.warmup_iters = warmup_iters
+        super().__init__(optimizer, last_epoch)
+
+    def get_lr(self):
+        warmup_factor = 1
+        if self.last_epoch < self.warmup_iters:
+            alpha = float(self.last_epoch) / self.warmup_iters
+            warmup_factor = self.warmup_factor * (1 - alpha) + alpha
+        return [
+            base_lr
+            * warmup_factor
+            * self.gamma ** bisect_right(self.milestones, self.last_epoch)
+            for base_lr in self.base_lrs
+        ]