Hello hamidreza_ramezani

I’m looking for an identifier that specifies a neural net’s layer in this.

That is ddp communication hook. It is used during the backward pass.

Also, I would like to know if this 2 loop iterates over all layers?

That loop iterates over all the parameters stored in the bucket. The parameters in the bucket are determined at construction time.
ddp doc Distributed Data Parallel — PyTorch 1.8.1 documentation 1
ddp communication hook doc DDP Communication Hooks — PyTorch 1.8.1 documentation 1