Method fallback

imperative/src/impl/operator.cpp:6–8

namespace imperative {

ValueRefList Operator::fallback(Span<ValueRef> inputs) const {
    mgb_throw(MegBrainError, "no fallback implementation for %s", to_string().c_str());
}

size_t Operator::register_type(std::type_index type) {
    auto& types = const_cast<std::vector<std::type_index>&>(registered_types());