remotes::install_github("tdhock/mlr3torch@fix-sampler")
rev_sampler_class <- torch::sampler(
  "MySampler",
  initialize = function(data_source) {
    print('init')
    self$data_source <- data_source
  },
  .iter = function() {
    count <<- 0L
    function() {
      if (count < length(self$data_source)) {
        idx <- length(self$data_source)-count
        count <<- count + 1L
        return(idx)
      }
      coro::exhausted()
    }
  },
  .length = function() {
    length(self$data_source)
  }
)
sonar_task <- mlr3::tsk("sonar")
mlp_learner <- mlr3torch::LearnerTorchMLP$new(task_type="classif")
mlp_learner$param_set$set_values(
  epochs=10,
  batch_size=20,
  sampler=rev_sampler_class)
mlp_learner$train(sonar_task)