[WIP]Streaming conformer pruned transducer

egs/librispeech/ASR/streaming_pruned_transducer_stateless/README.md

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+## wer with various right context
+
+related model and decoding result/log fils could be found:
+https://huggingface.co/GuoLiyong/icefall_streaming_prunned_transducer_stateless/tree/main/streaming_pruned_transducer_stateless/exp
+
+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
+
+
+
+## How latency is computed?
+
+latency = (subsampling factor * right_context + initialize_frames_need_by_subsampling_convs) * 10ms
+
+During which: subsmapling factor = 4 
+initialize_frames_need_by_subsampling_convs = 3
+
+To decode the first frame encoder out: 7 frams fbanks = subsampling_factor + initialize_frames_need_by_subsampling_convs are needed. 
+Once the deocding started, 4 frames fbank are needed per encoder_out frame.
+
+
+## Why does tailing dummy frames help?
+
+As 4 frames fbank are needed per encoder_out frame, suppose only 3(or 2,1) frames left, after a decoding process. 
+There will no encoder out frames corresponding to these 3 frames. 
+This may results in some "substitution/deletion errors" at the end. 
+By padding some dummy frames to the right, this problem could be solved to some extent.
+
+### Some Examples results:
+padding 0 frame|padding 20 frames
+--|--
+WITH ONE JUMP (ANDERS->ANDREWS) GOT OUT OF HIS (CHAIR->CHA)|WITH ONE JUMP (ANDERS->ANDREWS) GOT OUT OF HIS CHAIR
+COME WE'LL HAVE OUR COFFEE IN THE OTHER ROOM AND YOU CAN (SMOKE->SMO)|COME WE'LL HAVE OUR COFFEE IN THE OTHER ROOM AND YOU CAN SMOKE
+THINKING OF ALL THIS I WENT TO (SLEEP->SLEE)|THINKING OF ALL THIS I WENT TO SLEEP 
+STEAM UP AND CANVAS SPREAD THE SCHOONER STARTED (EASTWARDS->EASTWARD)|STEAM UP AND CANVAS SPREAD THE SCHOONER STARTED EASTWARDS
+
+### final Wers and detail error counts :
+*|wer|ins|del|sub
+--|--|--|--|--
+padding 0|5.60|329|283|2332
+padding 20 frames|5.52|329|282|2291
+
+Raw log files of previous table:
+```
+padding 0 frames:
+%WER = 5.60 
+Errors: 329 insertions, 283 deletions, 2332 substitutions, over 52576 reference words (49961 correct)
+
+padding 20 frames:
+%WER = 5.52
+Errors: 329 insertions, 282 deletions, 2291 substitutions, over 52576 reference words (50003 correct) 
+```
+
+

egs/librispeech/ASR/streaming_pruned_transducer_stateless/asr_datamodule.py

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+../transducer/asr_datamodule.py

egs/librispeech/ASR/streaming_pruned_transducer_stateless/beam_search.py

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+../pruned_transducer_stateless/beam_search.py