content: # Modified from https://github.com/facebookresearch/maskrcnn-benchmark/blob/master/maskrcnn_benchmark/solver/lr_scheduler.py
  from bisect import bisect_right
  
  import torch
  import math
  
  
  class WarmupMultiStepLR(torch.optim.lr_scheduler._LRScheduler):
      def __init__(
          self,
          optimizer,
          milestones,
          gamma=0.1,
          warmup_factor=1.0 / 3,
          warmup_iters=500,
          warmup_method="linear",
          last_epoch=-1,
      ):
          if not list(milestones) == sorted(milestones):
              raise ValueError(
                  "Milestones should be a list of" " increasing integers. Got {}",
                  milestones,
              )
  
          if warmup_method not in ("constant", "linear"):
              raise ValueError(
                  "Only 'constant' or 'linear' warmup_method accepted"
                  "got {}".format(warmup_method)
              )
          self.milestones = milestones
          self.gamma = gamma
          self.warmup_factor = warmup_factor
          self.warmup_iters = warmup_iters
          self.warmup_method = warmup_method
          super(WarmupMultiStepLR, self).__init__(optimizer, last_epoch)
  
      def get_lr(self):
          warmup_factor = 1
          if self.last_epoch < self.warmup_iters:
              if self.warmup_method == "constant":
                  warmup_factor = self.warmup_factor
              elif self.warmup_method == "linear":
                  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
          ]
  
  class HalfPeriodCosStepLR(torch.optim.lr_scheduler._LRScheduler):
      def __init__(
          self,
          optimizer,
          warmup_factor=1.0 / 3,
          warmup_iters=8000,
          max_iters=60000,
          warmup_method="linear",
          last_epoch=-1,
      ):
          if warmup_method not in ("constant", "linear"):
              raise ValueError(
                  "Only 'constant' or 'linear' warmup_method accepted"
                  "got {}".format(warmup_method)
              )
          self.warmup_factor = warmup_factor
          self.warmup_iters = warmup_iters
          self.max_iters = max_iters
          self.warmup_method = warmup_method
          super(HalfPeriodCosStepLR, self).__init__(optimizer, last_epoch)
  
      def get_lr(self):
          warmup_factor = 1
          if self.last_epoch < self.warmup_iters:
              if self.warmup_method == "constant":
                  warmup_factor = self.warmup_factor
              elif self.warmup_method == "linear":
                  alpha = float(self.last_epoch) / self.warmup_iters
                  warmup_factor = self.warmup_factor * (1 - alpha) + alpha
          else:
              warmup_factor = 0.5 * (math.cos(self.last_epoch / self.max_iters * math.pi) + 1)
          return [
              base_lr
              * warmup_factor
              for base_lr in self.base_lrs
          ]