Build n-gram language model for DeepSpeech2, and add inference interfaces insertable to CTC decoder.

[xinghai-sun]

• Train an Engish language model (Kneser-Ney smoothed 5-gram, with pruning), with KenLM toolkit, on cleaned text from the Common Crawl Repository. For detailed requirements please refer to DS2 paper.
• Add the training script into the DS2 trainer script.
• Add inference interfaces for this n-gram language model, insertable to [CTC-LM-beam-search]() for decoding.
• Keep in mind that the interfaces should be compatible with both English (word-based LM) and Madarin (character-based LM).
• Please work closely with the "Add CTC-LM-beam-search decoder" task.
• Refer to the DS2 design doc and update it when necessary.

[pkuyym]

@cxwangyi @kuke @xinghai-sun Hi, as mentioned in the paper, a language model has to be trained to improve the generating results and the LM is a critical component to ensure the performance. The language model is trained on texts crawled from commoncrawl.org using KenLM toolkit. However, we need more details to train such a language model. Any possible to get the trained language model or text dataset trained on?

[xinghai-sun]

1. 英文的语料应该很多，不一定拘泥于paper提到的语料，我们可以先小语料试，例如PTB。
2. n-gram LM训练的工具尽可能先用KenLM；如果不用，也尽可能保证 smooth的方法对齐或合理。
3. 重点关注model loading和inference的接口设计，并且做好和beam search decoder的联调。
4. 联系NLP或者SVAIL看看有没有现成的powerful LM model，中英文都问下，请@lcy-seso 协助下。

[wwfcnu]

1. 英文的语料应该很多，不一定拘泥于paper提到的语料，我们可以先小语料试，例如PTB。
2. n-gram LM训练的工具尽可能先用KenLM；如果不用，也尽可能保证 smooth的方法对齐或合理。
3. 重点关注model loading和inference的接口设计，并且做好和beam search decoder的联调。
4. 联系NLP或者SVAIL看看有没有现成的powerful LM model，中英文都问下，请@lcy-seso 协助下。

@lcy-seso 请问有现成可用的LM model可以用吗