Confusion about force of ghost atoms in lammps api

[lhycms]

I thought that Deepmd doesn't output the force on the ghost atoms, but it seems that there is a value assigned to the force on the ghost atom in Lammps extension(https://github.com/deepmodeling/deepmd-kit/blob/master/source/lmp/pair_deepmd.cpp). What confuse me is that how to calculate the force on ghost atoms?
Besides, why we still need to reverse communicate when we request a full neighbor list?

for (int ii = 0; ii < nall; ++ii) {
      for (int dd = 0; dd < 3; ++dd) {
        int new_idx = new_idx_map[ii];
        f[ii][dd] +=
            scale[1][1] * dforce[3 * new_idx + dd] * force_unit_cvt_factor;
        if (dtype[ii] < numb_types_spin && ii < nlocal) {
          fm[ii][dd] += scale[1][1] * dforce[3 * (new_idx + nlocal) + dd] /
                        (hbar / spin_norm[dtype[ii]]) * force_unit_cvt_factor;
        } else if (dtype[ii] < numb_types_spin) {
          fm[ii][dd] += scale[1][1] * dforce[3 * (new_idx + nghost) + dd] /
                        (hbar / spin_norm[dtype[ii]]) * force_unit_cvt_factor;
        }
      }

[njzjz]

The force on ghost atoms is still the negative gradient of total energy (in this processor):

$$\mathbf{F}_i = - \frac{\partial E}{\partial \mathbf{R}_i}$$