src/transformers/models/fsmt/modeling_fsmt.py/triu_onnx
def triu_onnx(x, diagonal=0):
    l = x.shape[0]
    arange = torch.arange(l, device=x.device)
    mask = arange.expand(l, l)
    arange = arange.unsqueeze(-1)
    if diagonal:
        arange = arange + diagonal
    mask = mask >= arange
    return x.masked_fill(mask == 0, 0)