Batched

[soumith]

This brings the rewritten, more efficient model from https://github.com/zackchase/mxnet-the-straight-dope/tree/master/chapter09_natural-language-processing

• On 0.2 this is about 50% faster on CPU (OMP_NUM_THREADS=1)
• On master this is about 3x faster on CPU (OMP_NUM_THREADS=1) On CUDA you'll see decent speedups as well.

This PR has to be carefully reviewed to make sure that the model before and after are doing the same thing (i've verified that the input / output shapes are all the same).

[dasguptar]

Hi @soumith :open_mouth:

Thanks for taking the time to send this PR. Unfortunately, after your previous PR, I kind of went back and looked at the model and realised I did many things in a dumb way.

Since this was the first model I had implemented in order to learn PyTorch, there was a bunch of unnecessary stuff, like F.torch.squeeze(tensor) instead of tensor.squeeze(). I went and tried to refactor the model myself yesterday, and optimised it a bit, reaching about ~2x speedup on CPU (5 minutes 30 seconds earlier to around 2 minutes 50 seconds now). I think some of the changes are similar to what you have done here, e.g. computing batched embeddings, combining linear layers, etc.

As of the latest commit, since the model file has changed, I cannot directly merge this PR, and I am quite inexperienced with rebasing and resolving conflicts. If and when you have the time, could you take a look at the current model.py, and decide whether to rebase the PR on current master, or if the current master is good enough?