TDT compute timestamps option and Extra Whitespace handling for SPE

[monica-sekoyan]

What does this PR do ?

This PR includes option for getting timestamps from TDT Decoder head, including on a segment level. This option is enabled for RNNT and CTC decodings as well. And also introduces workaround for getting tokens and id_s for extra whitespaces (even if the tokenizer was not trained in this way.) This is crucial for getting alignments in the future.

Collection: asr, common

Changelog

• Add specific line by line info of high level changes in this PR.

Usage

• To get timestamps from TDT decoding head, just change the decoding config of the model in the following way:

import nemo.collections.asr.models as nemo_asr
import omegaconf

tdt_asr_model = nemo_asr.EncDecRNNTBPEModel.from_pretrained(model_name='nvidia/parakeet-tdt_ctc-1.1b')

cfg = model.cfg.decoding
with omegaconf.open_dict(cfg):
    cfg['compute_timestamps']=True

tdt_asr_model.change_decoding_strategy(decoding_cfg=cfg)

• To control timestamp level manipulate the following field of decoding config:

cfg = model.cfg.decoding
with omegaconf.open_dict(cfg):
    cfg['compute_timestamps']=True
    cfg['rnnt_timestamp_type'] = 'all' # can be "char", "word", "segment" or "all"

tdt_asr_model.change_decoding_strategy(decoding_cfg=cfg)

• To make the SPE tokenizer (already trained) encode the text not ignoring occurring extra whitespaces (which is the default behaviour), modify the tokenizer in the following way:

model.tokenizer.ignore_extra_whitespaces = False

• To train the SPE tokenizer to make it ignore extra whitespaces both during training and further encoding, use this flag in the training script:

python scripts/tokenizers/process_asr_text_tokenizer.py --tokenizer=spe --spe_remove_extra_whitespaces

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Additional Information

• Related to # (issue)