RNN-Transducer Support (Beta)
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* 1 for &#39;import \*&#39; may pollute namespace This pull request **introduces 7 alerts** and **fixes 1** when merging 5cc3ab4049bd1a51bc0c424caa90a1c9ce99ef0d into ece60b02f10494d73b3ca109d2c4fa6ef556ec47 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-d51f2239ea14a1a71ff104bad3a3a2e58e25b664)

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* 8 for Unused import @redoctopus Thinking about it, the validation of configs is (kinda) part of my other PR here - https://github.com/NVIDIA/NeMo/pull/1328. Maybe lets tackle it in a separate PR? This pull request **introduces 8 alerts** when merging ebf1968a9e0a310882b8c484ae7a402a26ba13af into 61ab90646df5ce81928a68bcd5ba2a10c3c648fe - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-9e921249edd5cc69f4d40ba090ff00d9f0a5b0d8)

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* 8 for Unused import > @redoctopus Thinking about it, the validation of configs is (kinda) part of my other PR here - #1328. Maybe lets tackle it in a separate PR?

Yeah, having it in that PR sounds fine to me! Makes more sense, too; it'll be good to have it as something more inherent to NeMo rather than just ASR-specific. This pull request **introduces 7 alerts** when merging 6492b3952f5f9987bf3ec0c6fc84c74fcfcc33f1 into 99ed9c2ae5c840a8723f236530ccb1abe8c75377 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-8fc3eb36cbd9c21c439179bdfe90e55567c907ba)

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# Changelog
- Adds support for RNN-T based models
- Adds support for RNN-T BPE based models
- Adds basic configs (prototypes) for RNN-T based models.
- Adds examples/asr/speech_to_text_rnnt.py and examples/asr/speech_to_text_rnnt_bpe.py training scripts.
- Adds support for RNN-T loss based on https://github.com/HawkAaron/warp-transducer
- Add support for [Greedy] - `greedy`, `greedy_batch` and [Beam] - `beam`, `tsd` and `alsd` decoding support for RNNT models
- Detach *Decoding from WER computation so it should be easier to update to PTL 1.x
- Add experimental support for fused joint-loss-wer step in order to save memory by trading off compute speed.

# Todo
- [x] Model infrastructure
- [x] Basic config prototypes
- [x] Unittests
  - [x] Save restore RNNT and RNNTBPE models
  - [x] Change vocab for RNNT and RNNTBPE models
  - [x] Basic functionality test for RNNTJoint and RNNTDecoder
- [x] Cleanup commented code
- [x] Detach `RNNTDecoding` and `RNNTBPEDecoding` from PTL 1.x TensorMetric replacement.
- [x] Wrap the output of *Decoding to PTL 1.x TensorMetric replacement
- [x] Update configs to support PTL 1.x
- [ ] ~(Maybe) Factorize common elements of CTC and RNNT out into ASRCTCModel and ASRRNNTModel.~
- [ ] ~(Maybe) Refactor `rnn.py`~ 
- [ ] CI pipeline
- [x] Documentation
  - [x] RNNT Models
  - [x] RNNT BPE Models
  - [x] RNNT Loss
  - [x] RNNT Decoding + RNNT WER
  - [x] RNNT WPE Decoding + RNNT BPE WER
  - [x] RNNT Modules
  - [x] `rnn.py`
  - [x] GreedyRNNTInfer
  - [x] BeamRNNTInfer
  - [x] `rnnt_utils.py`
