I guess the idea behind the update rule for buf here(rmsprop) is to stay consistent with that for sgd with momentum.
buf.mul_(momentum).add_(1 - dampening, d_p)

SGD Momentum implementation
In pytoch implementation, the momentum is slightly changed, refer to sgd doc 1

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They are the same for Normal SGD and pytorch SGD when learning rate is fixed.
Hope this will be helpful.