Add method packs to the model

[egor-bogomolov]

1. Create new dataset with packs of several methods instead of single methods.
2. Change data loading pipeline.
3. Update model, give each method in a pack attention weight.

[egor-bogomolov]

Support of method packs is implemented. It has some problems with memory efficiency though.
So far, for packs of size 16 sampled from the middle third of programmers history it has accuracy close to 1.

[egor-bogomolov]

Next steps:

1. Fix memory problems
2. Make train and test packs more distinct: move from different files to different packages or sample from different parts of history
3. Extract methods with highest/lowest attention for each programmer in readable format
4. Explore influence of pack size and other hyperparameters on accuracy