You are correct that packed sequence is for lstm.
If you have 100000 indices, how will you back propogate through time? I would use some kind of a sequence window (say 10), and pass at most 10 inputs at a time. You could also do it like in the awd-lstm paper where the bptt window is random (see https://arxiv.org/pdf/1708.02182.pdf 3 for more details).
Not sure if this answers your question though.