Add CTC-LM-beam-search decoder.

[xinghai-sun]

• The current paddle.layer.beam_search decoder only supports frame-wise decoding, not CTC decoding.
• Add paddle.layer.ctc_beam_search to support CTC beam search decoding, with external language model scorer and word-count scorer. Please refer to Prefix Beam Search, CTC paper, and DS2 paper
• It would be better to re-design the above layers, and provide a more general interfaces for beam search with any external custom scoring function.
• Keep in mind that it would be better if it is compatible with both English (word-based LM) and Madarin (character-based LM).
• Please add a design-doc here first, illustrating:
  • the algorithm of CTC prefix beam search with LM and WordCount;
  • how to design a general beam search interface.
• This can also be used to improve the current OCR model performance.