Stateless decoder for RNN-T

[hainan-xv]

What does this PR do ?

This PR adds stateless decoder support for RNN-T ASR models.

Collection: [Note which collection this PR will affect]

Changelog

• adds a stateless decoder class for RNN-T ASR models.
• adds another set of asr model tests for partial decoding.
• small bus fixes for asr greedy decode during partial decoding

Usage

To launch a RNN-T model with a stateless decoder, change the training script by adding the following lines with recommended settings,

model.decoder._target_=nemo.collections.asr.modules.RNNTStatelessDecoder \
+model.decoder.context_size=2 \
model.decoder.normalization_mode=layer \
~model.decoder.random_state_sampling \

PR Type:

• [ x ] New Feature
• [ ] Bugfix
• [ ] Documentation

[lgtm-com[bot]]

This pull request introduces 1 alert when merging 452dac89710902e0fb16b171c75d6e8831b78ee1 into f921ebe0436e55f7547b183ca83a623f6678422d - view on LGTM.com

new alerts:

• 1 for Unreachable code

[titu1994]

Needs formatting of code + some docstrings. Other stuff looks quite good.

[hainan-xv]

Updated PR addressing Som's comments.

[lgtm-com[bot]]

This pull request introduces 1 alert when merging 707fa760a854c967fefb801b7e3adb4a0ad6b4cb into 3ba267290927b1a2e78d82c7afbc538874c01bcc - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com[bot]]

This pull request introduces 1 alert when merging c695911f046afae9f8e4eb59c617166386ba022c into 4a1fcc62971de89db283334c0dd4aa112e473cbf - view on LGTM.com

new alerts:

• 1 for Unused import

[titu1994]

Forgot to mention, lets add a directory for stateless transducers in conf, with subdirectory for Conformer. So that people can more easily find and train such models c