TDT model pull request
# What does this PR do ?

Token-and-Duration Transducer (TDT, https://arxiv.org/abs/2304.06795) implementation.

**Collection**: 
ASR

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage

To train a TDT model, use config file at examples/asr/conf/conformer/tdt/conformer_tdt_bpe.yaml. You can change the $durations parameter to customize your model, e.g.

```
durations=[0,1,2,3]
num_durations=4  # this needs to match the previous durations variable.

python examples/asr/asr_transducer/speech_to_text_rnnt_bpe.py \
        [options similar to launching conventional Transducer models] \
        model.loss.tdt_kwargs.fastemit_lambda=0.001 \
        model.loss.tdt_kwargs.omega=0.1 \
        model.model_defaults.tdt_durations=$durations \
        model.model_defaults.num_tdt_durations=$num_durations \
        model.decoding.strategy='greedy'   # Greedy decoding is recommended to get more accurate validation WERs during training. It's already set in the config file as default. We list it here to emphasize its importance.

```

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**PR Type**:
- [x] New Feature
- [ ] Bugfix
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# Additional Information
* Related to # (issue)
