If you maintain the same batch size between single GPU and DP/DDP, according to your calculations you do not need to adjust LR ?
PS:
In DDP grads are averaged: https://pytorch.org/docs/master/generated/torch.nn.parallel.DistributedDataParallel.html#torch.nn.parallel.DistributedDataParallel 99

During the backwards pass, gradients from each node are averaged.

PPS: https://arxiv.org/pdf/1706.02677.pdf 46

Linear Scaling Rule: When the minibatch size is multiplied by k, multiply the learning rate by k.