Merge pull request #2255 from xinghai-sun/ds2_design_doc

Add design doc for DeepSpeech2 on PaddlePaddle.

doc/design/speech/README.MD

+# DeepSpeech2 on PaddlePaddle: Design Doc 
+
+We are planning to build Deep Speech 2 (DS2) \[[1](#references)\], a powerful Automatic Speech Recognition (ASR) engine,  on PaddlePaddle. For the first-stage plan, we have the following short-term goals:
+
+- Release a basic distributed implementation of DS2 on PaddlePaddle.
+- Contribute a chapter of Deep Speech to PaddlePaddle Book.
+
+Intensive system optimization and low-latency inference library (details in \[[1](#references)\]) are not yet covered in this first-stage plan.
+
+## Table of Contents
+
+- [Tasks](#tasks)
+- [Task Dependency](#task-dependency)
+- [Design Details](#design-details)
+    - [Overview](#overview)
+    - [Row Convolution](#row-convolution)
+    - [Beam Search With CTC and LM](#beam-search-with-ctc-and-lm)
+- [Future Work](#future-work)
+- [References](#references)
+
+## Tasks
+
+We roughly break down the project into 14 tasks:
+
+1. Develop an **audio data provider**:
+	- Json filelist generator.
+	- Audio file format transformer.
+	- Spectrogram feature extraction, power normalization etc.
+	- Batch data reader with SortaGrad.
+	- Data augmentation (optional).
+	- Prepare (one or more) public English data sets & baseline.
+2. Create a **simplified DS2 model configuration**:
+   - With only fixed-length (by padding) audio sequences (otherwise need *Task 3*).
+	- With only bidirectional-GRU (otherwise need *Task 4*).
+	- With only greedy decoder (otherwise need *Task 5, 6*).
+3. Develop to support **variable-shaped** dense-vector (image) batches of input data.
+   - Update `DenseScanner` in `dataprovider_converter.py`, etc.
+4. Develop a new **lookahead-row-convolution layer** (See \[[1](#references)\] for details):
+   - Lookahead convolution windows.
+   - Within-row convolution, without kernels shared across rows.
+5. Build KenLM **language model** (5-gram) for beam search decoder:
+   - Use KenLM toolkit.
+   - Prepare the corpus & train the model.
+   - Create infererence interfaces (for Task 6).
+6. Develop a **beam search decoder** with CTC + LM + WORDCOUNT:
+   - Beam search with CTC.
+   - Beam search with external custom scorer (e.g. LM).
+   - Try to design a more general beam search interface.
+7. Develop a **Word Error Rate evaluator**:
+   - update `ctc_error_evaluator`(CER) to support WER.
+8. Prepare internal dataset for Mandarin (optional):
+    - Dataset, baseline, evaluation details.
+    - Particular data preprocessing for Mandarin.
+    - Might need cooperating with the Speech Department.
+9. Create **standard DS2 model configuration**:
+   - With variable-length audio sequences (need *Task 3*).
+	- With unidirectional-GRU + row-convolution (need *Task 4*).
+	- With CTC-LM beam search decoder (need *Task 5, 6*).
+10. Make it run perfectly on **clusters**.
+11. Experiments and **benchmarking** (for accuracy, not efficiency):
+    - With public English dataset.
+    - With internal (Baidu) Mandarin dataset (optional).
+12. Time **profiling** and optimization.
+13. Prepare **docs**.
+14. Prepare PaddlePaddle **Book** chapter with a simplified version.
+
+## Task Dependency
+
+Tasks parallelizable within phases:
+
+Roadmap     | Description                               | Parallelizable Tasks 
+----------- | :------------------------------------     | :--------------------
+Phase I	    | Simplified model & components             | *Task 1* ~ *Task 8*
+Phase II    | Standard model & benchmarking & profiling | *Task 9* ~ *Task 12*
+Phase III   | Documentations                            | *Task13* ~ *Task14*
+
+Issue for each task will be created later. Contributions, discussions and comments are all highly appreciated and welcomed!
+
+## Design Details
+
+### Overview
+
+Traditional **ASR** (Automatic Speech Recognition) pipelines require great human efforts devoted to elaborately tuning multiple hand-engineered components (e.g. audio feature design, accoustic model, pronuncation model and language model etc.). **Deep Speech 2** (**DS2**) \[[1](#references)\], however, trains such ASR models in an end-to-end manner, replacing most intermediate modules with only a single deep network architecture. With scaling up both the data and model sizes, DS2 achieves a very significant performance boost.
+
+Please read Deep Speech 2 \[[1](#references),[2](#references)\] paper for more background knowledge.
+
+The classical DS2 network contains 15 layers (from bottom to top):
+
+- **Two** data layers (audio spectrogram, transcription text)
+- **Three** 2D convolution layers
+- **Seven** uni-directional simple-RNN layers
+- **One** lookahead row convolution layers
+- **One** fully-connected layers
+- **One** CTC-loss layer
+
+<div align="center">
+<img src="image/ds2_network.png" width=350><br/>
+Figure 1. Archetecture of Deep Speech 2 Network.
+</div>
+
+We don't have to persist on this 2-3-7-1-1-1 depth \[[2](#references)\]. Similar networks with different depths might also work well. As in \[[1](#references)\], authors use a different depth (e.g. 2-2-3-1-1-1) for final experiments.
+
+Key ingredients about the layers:
+
+- **Data Layers**: 
+   - Frame sequences data of audio **spectrogram** (with FFT).
+   - Token sequences data of **transcription** text (labels). 
+   - These two type of sequences do not have the same lengthes, thus a CTC-loss layer is required.
+- **2D Convolution Layers**: 
+   - Not only temporal convolution, but also **frequency convolution**. Like a 2D image convolution, but with a variable dimension (i.e. temporal dimension).
+   - With striding for only the first convlution layer.
+   - No pooling for all convolution layers.
+- **Uni-directional RNNs** 
+	- Uni-directional + row convolution: for low-latency inference.
+	- Bi-direcitional + without row convolution: if we don't care about the inference latency.
+- **Row convolution**:
+	- For looking only a few steps ahead into the feature, instead of looking into a whole sequence in bi-directional RNNs.
+	- Not nessesary if with bi-direcitional RNNs. 
+	- "**Row**" means convolutions are done within each frequency dimension (row), and no convolution kernels shared across.
+- **Batch Normalization Layers**:
+   - Added to all above layers (except for data and loss layer).
+   - Sequence-wise normalization for RNNs: BatchNorm only performed on input-state projection and not state-state projection, for efficiency consideration.
+ 
+
+Required Components                     | PaddlePaddle Support                      | Need to Develop
+:-------------------------------------  | :--------------------------------------   | :-----------------------
+Data Layer I (Spectrogram)	            | Not supported yet.                        |  TBD (Task 3)
+Data Layer II (Transcription)           | `paddle.data_type.integer_value_sequence` | -
+2D Convolution Layer                    | `paddle.layer.image_conv_layer`           | -
+DataType Converter (vec2seq)            | `paddle.layer.block_expand`               | -
+Bi-/Uni-directional RNNs                | `paddle.layer.recurrent_group`            | -
+Row Convolution Layer                   | Not supported yet.                        | TBD (Task 4)
+CTC-loss Layer                          | `paddle.layer.warp_ctc`                   | -
+Batch Normalization Layer               | `paddle.layer.batch_norm`                 | -
+CTC-Beam search                         | Not supported yet.                        | TBD (Task 6)
+
+### Row Convolution
+
+TODO by Assignees
+
+### Beam Search with CTC and LM
+
+TODO by Assignees
+
+## Future Work
+
+- Efficiency Improvement
+- Accuracy Improvement
+- Low-latency Inference Library
+- Large-scale benchmarking
+
+## References
+
+1. Dario Amodei, etc., [Deep Speech 2 : End-to-End Speech Recognition in English and Mandarin](http://proceedings.mlr.press/v48/amodei16.pdf). ICML 2016.
+2. Dario Amodei, etc., [Deep Speech 2 : End-to-End Speech Recognition in English and Mandarin](https://arxiv.org/abs/1512.02595). 	arXiv:1512.02595.