Pytorch: Speed up get_log_probs function

[binhvq]

Hi lopuhin.
Thanks for your sharing.
I used your model to predict the next word, but I found the forecast speed relatively slow, probably because of the lm.inference.get_log_probs function to predict the probability of all both words in sentences. Meanwhile, the problem of predicting the next word only requires the probability of the last word.

[lopuhin]

I found the forecast speed relatively slow

Thanks for feedback. Did you find it slow compared to other similar models / implementations?

Meanwhile, the problem of predicting the next word only requires the probability of the last word.

Right. But we still need to process all previous words. I see that we could avoid doing softmax for all but the last word, not sure how much difference will it bring.

[lopuhin]

FWIW there is a big speedup in text generation here 4c18649 - this speeds up generation of multiple tokens, while the single token generation has the same speed.