WIP: BPE Training with k2 ctc loss

[pkufool]

FYI @glynpu

My current result are as follows (without transformer decoder, say att_rate = 0.0):

decoding model	WER % on test-clean	WER % on test-other
avg epoch 18 - 28	11.94	26.78
avg epoch 9 - 13	31.65	58.88

[pkufool]

There was a bpe_run.sh in master branch, posted by Liyong in #217, it only did decoding things, so I rename it to bpe_decode.sh. All the training and decoding pipeline are in bpe_run.sh script in this PR.

[pkufool]

We will discuss BPE training with k2 ctc loss in this PR #219