Indexing with negative indices failed

[marty1885]

98 handles negative steps. But doesn't handle negative indices.

This fails

Tensor q = t.view({3});
Tensor r = t.view({-1});
if(r.has_value()) {
    std::cout << r << std::endl;
    CHECK(q.isSame(r));
}

[marty1885]

solved in #124