Fix QubitVector::check_dimension to use only the data_size_ field

[gadial]

Summary

Fixes a bug where QubitVector::check_dimension attempted to access non-existing fields.

Fixes #2284

Details and comments

Originally, QubitVector::check_dimension was defined as

template <class data_t>
void QubitVector<data_t>::check_dimension(const QubitVector &qv) const {
  if (size_ != qv.size_) {
    std::stringstream ss;
    ss << "QubitVector: vectors are different shape ";
    ss << size_ << " != " << qv.size_;
    throw std::runtime_error(ss.str());
  }
}

In 502ffcd this was changed to

template <typename data_t>
void QubitVector<data_t>::check_dimension(const QubitVector &qv) const {
  if (data_size_ != qv.size_) {
    std::string error = "QubitVector: vectors are different shape " +
                         std::to_string(data_size_) + " != " +
                         std::to_string(qv.num_states_);
    throw std::runtime_error(error);
  }
}

Along with a change of the size_ field to data_size_. It is unclear why qv.size_ remained (maybe the idea was to use qv.size()?) nor what is the meaning of qv.num_states_ (since no such field or similar getter was added). It seems to me that changing to data_size_ retains the original meaning.