Based on this 14, this 11, and this 13 code snippets you should be able to add new parameter groups via:
 auto& params_groups = optimizer.param_groups();
 params_groups.push_back(OptimizerParamGroup(params));

and set the learning rate via:
static_cast<torch::optim::AdamOptions&>(adam_optimizer.param_groups()[0].options()).lr()

for Adam (you might want to change it to the appropriate optimizer type).