import torch


def subsequent_mask(
        size: int,
        device: torch.device = torch.device("cpu"),
) -> torch.Tensor:
    """Create mask for subsequent steps (size, size).

    This mask is used only in decoder which works in an auto-regressive mode.
    This means the current step could only do attention with its left steps.

    In encoder, fully attention is used when streaming is not necessary and
    the sequence is not long. In this  case, no attention mask is needed.

    When streaming is need, chunk-based attention is used in encoder. See
    subsequent_chunk_mask for the chunk-based attention mask.

    Args:
        size (int): size of mask
        str device (str): "cpu" or "cuda" or torch.Tensor.device
        dtype (torch.device): result dtype

    Returns:
        torch.Tensor: mask

    Examples:
        >>> subsequent_mask(3)
        [[1, 0, 0],
         [1, 1, 0],
         [1, 1, 1]]
    """
    arange = torch.arange(size, device=device)
    mask = arange.expand(size, size)
    arange = arange.unsqueeze(-1)
    mask = mask <= arange
    return mask