static bool hasContiguousSubspace(TensorList tl) {
// true if all the non-null tensors are adjacent
auto isDefined = [](const Tensor & tensor){ return tensor.defined(); };
auto isNull = [](const Tensor & tensor){ return !tensor.defined(); };
auto start = std::find_if(tl.begin(), tl.end(), isDefined);
auto stop = std::find_if(tl.rbegin(), tl.rend(), isDefined);
auto it = std::find_if(start, stop.base(), isNull);
return it == stop.base();
}
// ArrayRef is not covariant, which means there is no
// implicit conversion between TensorList (aka ArrayRef<Tensor>)
// and ArrayRef<Variable>. What we do instead is manually
// construct a variable_list, which itself is implicitly convertible
// into an ArrayRef<Variable> (but don't return an ArrayRef<Variable>;
// ArrayRef is non-owning!)
static variable_list cast_tensor_list(const TensorList& tensors) {
// TODO: Eliminate the intermediate vector allocation
return variable_list(tensors.begin(), tensors.end());
}