Finds sympy.tensor.indexed.Idx instances and returns them.

def get_idxs(exprs):
    """
    Finds sympy.tensor.indexed.Idx instances and returns them.
    """
    idxs = set()
    for expr in (exprs):
        for i in expr.find(sympy.Idx):
            idxs.add(i)
    return sorted(idxs, key=str)