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supercarrot | 9 years ago
For example, instead of learning the sequence labeling as a sequence of n decisions (where n is the length of the sequence) you can learn sequence labeling as a sequence of 3n+1 decisions where you make 3 decisions for each sequence element and after 3n decision pick one out of three decision streams that minimizes loss using an extra decision. (when inference is done then the classifier will, hopefully, pick the stream that minimizes test loss).
This simulates a beam search and can be done during learning and inference and is probably more effective than picking confidence scores of particular decisions and keeping a beam of most confident partial sequences.
Bean search is a heuristic thing that improves performance and is done mostly to allow you to correct mistakes you made at the beginning of the process.
http://arxiv.org/abs/1603.06042
they illustrate the problem well (label bias).
the question remains the same, for example, in the paper above they approximate the partition function of CRFs with a beam but get superior results to other structured prediction methods.
kastnerkyle|9 years ago
At least in NMT, enumerating the possible decisions as 3n + 1 is almost impossible since the softmax size is generally the memory bottleneck in training - and a bigger vocabulary is typically a huge win. It is more feasible in speech, but often your labels are themselves triphones and you end up with a pretty large vocabulary too.
Figuring out how to get RNNs closer to on par with DNN-HMMs with Viterbi decoding (or full on sequence training [3]) through something like "deep fusion" with a language model (or something else) is something I am very interested in.
[0] http://arxiv.org/abs/1312.6082
[1] http://arxiv.org/abs/1511.06456
[2] https://arxiv.org/abs/1511.04868
[3] http://www.danielpovey.com/files/2013_interspeech_dnn.pdf
supercarrot|9 years ago
This is exactly what they do with the dependency parser I've cited, so your opinion is definitely valid. Although their approach is not general, given the fact that they approximate hamming loss with log-loss and again make it work only on sequences.
http://arxiv.org/abs/1502.02206
paper above also has a very good analysis on how to remove the search component of the inference and allow linear time complexity with competitive results.
consistent (as it is used in the machine learning theory of reductions) reduction from structured learning to multiclass classification seems to be possible. I just haven't seen anyone couple the learning procedure with neural networks. (Daume did mention they trained RNNs with the reductionist approach but seems that the code didn't make it to vowpal wabbit).
the approach above works with any loss you want (from F-score to any weird thing you might think of), the loss doesn't have to decompose over the structure (one can just announce the loss after the labelling is done and learn from that loss), it can work on any kind of structure, from images to sequences to documents for translation. it can also use a O(log n) consistent reduction of multiclass classification if speed is of the issue and if number of classes is large. It can easily work as an online method too, not requiring the full structured input.
for example, simple sequence tagging works (depending on the number of possible labels) around 500k tokens per second :D word count is only 2-4 times faster than that :D
there still aren't any papers using the above consistent reduction in the framework of NNs but I guess they'll soon be coming.