Add some variable sequence length LSTM benchmarks (#42)
Added three benchmarks:
1) pack_sequence + pytorch native LSTM
2) custom JIT LSTM
3) python LSTM (in other words, the JIT LSTM without torchscript)
fastrnns.bench / fastrnns.test now take a '--variable_lstms' argument to
turn on benchmarking / testing for them. It's not enabled by default
because benchmarks (2) and (3) are pretty slow (on the order of
magnitude of seconds) vs 10's of ms for (1).
