[ci skip] NeMo Documentation for PyTorch Lightning

[ericharper]

What does this PR do?

This PR adds Conversational AI - NeMo to the PyTorch Lightning documentation.

NeMo is a Conversational AI toolkit that has everything needed to build ASR, NLP, and TTS models. Every NeMo Model is a LightningModule and uses the Trainer for multi-GPU and multi-node training. We use Hydra for configuring both NeMo Models and the PyTorch LightningTrainer.

The documentation shows how to install NeMo, explains the core concepts, and gives a detailed example for each domain (ASR/NLP/TTS) that shows how to use NeMo Models, Pytorch Lightning, and Hydra for training and inference.

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[williamFalcon]

thanks! let's hold off on this for a few days until we finalize the structure of our docs. we need to see where we'll feature our partner frameworks

[codecov]

Codecov Report

Merging #3707 into master will decrease coverage by 1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #3707    +/-   ##
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- Coverage      88%     88%    -1%     
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  Files         108     108            
  Lines        7745    7682    -63     
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- Hits         6843    6742   -101     
- Misses        902     940    +38