[WIP]Streaming conformer pruned transducer

[glynpu]

A streaming conformer training/decoding/stimulate_streaming decoding pipeline modified from pruned_transducer_stateless;
The model trained with full librispeech by 50 epochs.

decoding with ctc greedy search:

right_context	1	8	16	32	64	full
latency	0.07s	0.35s	0.67s	1.31s	2.59s	*
test_clean	5.60	4.00	3.76	3.75	3.65	3.28
+20 tailing dummy frames	5.52	3.98	3.75	3.75	3.65	3.28
simulate streaming with chunk_by_chunk decoding	5.52	3.98	3.75	3.75	3.65	3.28
test_other	14.07	10.69	9.80	9.48	9.01	8.05
+20 tailing dummy frames	14.00	10.69	9.80	9.48	9.0	8.04
simulate streaming with chunk_by_chunk decoding	14.00	10.69	9.80	9.48	9.0	8.04

Other information could be found in the streaming_pruned_transducer_stateless/README.md in this pr.

[danpovey]

nice! will look to-morrow.