Update LM and namespace

[cgraywang]

Description

1, update LM to make the StandardRNN forward and AWDRNN forward align.
2, enable StandardRNN use beta L2 regularization.
3, add small AWDRNN model pretrained setting and results.
4, update LM example and script's namespace according to the discussion.

Checklist

Essentials

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[cgraywang]

@szha please help to upload the pretrained model, it looks like I zip the model in a wrong format... Thanks.

The original model is here:
https://drive.corp.amazon.com/personal/chgwang/pretrained_models_and_vocab/awd_lstm_lm_600_wikitext-2-7894a046f.params

[cgraywang]

@leezu @szha
Please also help to take a look at:
#60

[mli]

Job PR-53/8 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-53/8/index.html