def build_detection_train_loader(
    dataset, *, mapper, sampler=None, total_batch_size, aspect_ratio_grouping=True, num_workers=0
):
    """
    Build a dataloader for object detection with some default features.
    This interface is experimental.

    Args:
        dataset (list or torch.utils.data.Dataset): a list of dataset dicts,
            or a map-style pytorch dataset. They can be obtained by using
            :func:`DatasetCatalog.get` or :func:`get_detection_dataset_dicts`.
        mapper (callable): a callable which takes a sample (dict) from dataset and
            returns the format to be consumed by the model.
            When using cfg, the default choice is ``DatasetMapper(cfg, is_train=True)``.
        sampler (torch.utils.data.sampler.Sampler or None): a sampler that
            produces indices to be applied on ``dataset``.
            Default to :class:`TrainingSampler`, which coordinates a random shuffle
            sequence across all workers.
        total_batch_size (int): total batch size across all workers. Batching
            simply puts data into a list.
        aspect_ratio_grouping (bool): whether to group images with similar
            aspect ratio for efficiency. When enabled, it requires each
            element in dataset be a dict with keys "width" and "height".
        num_workers (int): number of parallel data loading workers

    Returns:
        torch.utils.data.DataLoader: a dataloader. Each output from it is a
            ``list[mapped_element]`` of length ``total_batch_size / num_workers``,
            where ``mapped_element`` is produced by the ``mapper``.
    """
    if isinstance(dataset, list):
        dataset = DatasetFromList(dataset, copy=False)
    if mapper is not None:
        dataset = MapDataset(dataset, mapper)
    if sampler is None:
        sampler = TrainingSampler(len(dataset))
    assert isinstance(sampler, torch.utils.data.sampler.Sampler)
    return build_batch_data_loader(
        dataset,
        sampler,
        total_batch_size,
        aspect_ratio_grouping=aspect_ratio_grouping,
        num_workers=num_workers,
    )

    Build a dataloader for object detection with some default features.
    This interface is experimental.

    Args:
        dataset (list or torch.utils.data.Dataset): a list of dataset dicts,
            or a map-style pytorch dataset. They can be obtained by using
            :func:`DatasetCatalog.get` or :func:`get_detection_dataset_dicts`.
        mapper (callable): a callable which takes a sample (dict) from dataset and
            returns the format to be consumed by the model.
            When using cfg, the default choice is ``DatasetMapper(cfg, is_train=True)``.
        sampler (torch.utils.data.sampler.Sampler or None): a sampler that
            produces indices to be applied on ``dataset``.
            Default to :class:`TrainingSampler`, which coordinates a random shuffle
            sequence across all workers.
        total_batch_size (int): total batch size across all workers. Batching
            simply puts data into a list.
        aspect_ratio_grouping (bool): whether to group images with similar
            aspect ratio for efficiency. When enabled, it requires each
            element in dataset be a dict with keys "width" and "height".
        num_workers (int): number of parallel data loading workers

    Returns:
        torch.utils.data.DataLoader: a dataloader. Each output from it is a
            ``list[mapped_element]`` of length ``total_batch_size / num_workers``,
            where ``mapped_element`` is produced by the ``mapper``.
    
datasets/build.py