1. For a 1D system, as bulk, you would use 1x1x7 (or similar), but for a device you would use 1x1x1
2. The keywords are almost self-explanatory.
"symmetrize" is the same keyword as always, and means that in the output, the dynamical matrix is made symmetric (due to numerical noise, it may not be, otherwise)
"constrain_electrodes" should always be True for inelastic transmission, the electron-phonon coupling is only computed in the scattering region, but set it to False for phonon transmission
"use_equivalent_bulk" means that the dynamical matrix is computed for the central region treated as a bulk system. This is a lot cheaper than computing all forces in device mode.
