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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def cat(tensors, dim=0):
 """ Efficient version of torch.cat that avoids a copy if there is only a single element in a list """

    assert isinstance(tensors, (list, tuple))
    if len(tensors) == 1:
        return tensors[0]
    return torch.cat(tensors, dim)