Creates learning rate schedulers
lr_scheduler(
  classname = NULL,
  inherit = LRScheduler,
  ...,
  parent_env = parent.frame()
)optional name for the learning rate scheduler
an optional learning rate scheduler to inherit from
named list of methods. You must implement the get_lr()
method that doesn't take any argument and returns learning rates
for each param_group in the optimizer.
passed to R6::R6Class().