import torch


def reduce(to_reduce: torch.Tensor, reduction: str) -> torch.Tensor:
    """
    Reduces a given tensor by a given reduction method

    Args:
        to_reduce : the tensor, which shall be reduced
       reduction :  a string specifying the reduction method ('elementwise_mean', 'none', 'sum')

    Return:
        reduced Tensor

    Raise:
        ValueError if an invalid reduction parameter was given
    """
    if reduction == 'elementwise_mean':
        return torch.mean(to_reduce)
    if reduction == 'none':
        return to_reduce
    if reduction == 'sum':
        return torch.sum(to_reduce)
    raise ValueError('Reduction parameter unknown.')