An off-duty, NYPD undercover officer was among the pack of motorcyclists who chased a Financial District family up the West Side Highway — and he stood by as the dad was hauled from his car and beaten, The Post has learned. The unidentified officer only came forward Wednesday night, four days after the caught-on-video biker predation, and is described as a motorcycle hobbyist who rides with Front Line Soldiers, a New Rochelle-based group that also counts several other cops among its members, a source said.