Transformer LM lattice rescoring

[greed2411]

To be updated.

[pskrunner14]

Implement PyTorch Transformer LM lattice rescoring in the Decoder class of kaldi-serve core C++ library. Please find a similar implementation for RNN LM lattice rescoring here. The idea here is to be able to rescore the n-gram LM lattice using a more sophisticated model like a Transformer to acheive greater accuracy.

Relevant links:

1. PyTorch NN Lattice Rescoring - script
2. NN LM Rescore - Source
3. LM Lattice Rescore - Source