add user guide for ant-xgboost (#772)

Author: sperlingxx

doc/ant-xgboost_on_sqlflow_design.md renamed to doc/ant-xgboost_design.md

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+# _user guide:_ Ant-XGBoost on sqlflow
+
+## Overview
+
+[Ant-XGBoost](https://github.com/alipay/ant-xgboost) is fork of [dmlc/xgboost](https://github.com/dmlc/xgboost), which is maintained by active contributors of dmlc/xgboost in Alipay Inc.
+
+Ant-XGBoost extends `dmlc/xgboost` with the capability of running on Kubernetes and automatic hyper-parameter estimation. 
+In particular, Ant-XGBoost includes `auto_train` methods for automatic training and introduces an additional parameter `convergence_criteria` for generalized early stopping strategy.
+See supplementary section for more details about automatic training and generalized early stopping strategy.
+
+## Tutorial
+We provide an [interactive tutorial](../example/jupyter/tutorial_antxgb.ipynb) via jupyter notebook, which can be run out-of-the-box in [sqlflow playground](https://play.sqlflow.org).
+If you want to run it locally, you need to install SQLFlow first. You can learn how to install sqlflow at [here](../doc/installation.md).
+
+## Concepts
+### Estimators
+We provide various XGBoost estimators for better user experience.
+All of them are case-insensitive and sharing same prefix `xgboost`. They are listed below.
+
+* xgboost.Estimator
+
+  General estimator, with this `train.objective` should be defined explicitly.
+  
+* xgboost.Classifier 
+    
+  Estimator for classification task, works with `train.num_class`. Default value is binary classification.
+  
+* xgboost.BinaryClassifier 
+    
+  Estimator for binary classification task, the value of `train.objective` is `binary:logistic`.
+
+* xgboost.MultiClassifier 
+    
+  Estimator for multi classification task, the value of `train.objective` is `multi:softprob`. It should work with `train.num_class` > 2.
+
+* xgboost.Regressor 
+    
+  Estimator for regression task, the value of `train.objective` is `reg:squarederror`(`reg:linear`). 
+
+### Columns
+
+* Feature Columns
+
+    For now, two kinds of feature columns are available.
+    
+    First one is `dense schema`, which concatenate numeric table columns transparently, such as `COLUMN f1, f2, f3, f4`.
+    
+    Second one is `sparse key-value schema`, which received LIBSVM style key-value string formatted like `$k1:$v1,$k2:$v2,...`.
+    This schema is decorated with keyword `SPARSE`, such as `COLUMN SPARSE(col1)`.
+
+* Label Column
+  
+   Following general sqlflow syntax, label clause of AntXGBoost is formatted in  `LABEL $label_col`. 
+
+* Group Column
+
+    In training mode, group column can be declared in a separate column clause. Group column is identified by keyword `group`, such as `COLUMN ${group_col} FOR group`.
+
+* Weight Column
+    
+    As group column schema, weight column is identified by keyword `weight`, such as `COLUMN ${weight_col} FOR weight`.
+
+* Result Columns
+    
+    Schema of straightforward result (class_id for classification task, score for regression task) is following general sqlflow syntax(`PREDICT ${output_table}.${result_column}`).
+
+    In addition, we also provide supplementary information of XGBoost prediction. They can be configured with `pred.attributes`.
+
+    * append columns
+
+        Columns of prediction data table which need to be appended into result table, such as id_col, label_col.
+        
+        The syntax is `pred.append_columns = [$col1, $col2, ...]`. 
+    
+    * classification probability
+        
+        Probability of the chosen class, which only work in classification task.
+
+        The syntax is `pred.prob_column = ${col}`.
+    
+    * classification detail
+        A json string who holds the probability distribution of all classes, formatted like `{$class_id:$class_prob,...}`. Only work in classification task.
+
+        The syntax is `pred.detail_column = ${col}`.
+    
+    * encoding of leaf indices 
+        
+        Predicted leaf index in each tree, they are joined orderly into a string with format `$id_1,$id_2,...`.
+
+        The syntax is `pred.encoding_column = ${col}`.
+
+### Attributes
+
+There exists two kinds of attributes, `train.attributes` and `pred.attrbutes`.
+`train.attributes`, which starts with prefix `train.`, only work in training mode.
+`pred.attributes`, which starts with prefix `pred.`, only work in prediction mode.
+
+All attributes are optional except `train.objective` must be defined when training with `xgboost.Estimator`.
+
+#### Available train.attributes
+
+* [General Params](https://xgboost.readthedocs.io/en/latest/parameter.html#general-parameters)
+    * train.booster
+    * train.verbosity
+
+* [Tree Booster Params](https://xgboost.readthedocs.io/en/latest/parameter.html#parameters-for-tree-booster)
+    * train.eta
+    * train.gamma
+    * train.max_depth
+    * train.min_child_weight
+    * train.max_delta_step
+    * train.subsample
+    * train.colsample_bytree
+    * train.colsample_bylevel
+    * train.colsample_bynode
+    * train.lambda
+    * train.alpha
+    * train.tree_method
+    * train.sketch_eps
+    * train.scale_pos_weight
+    * train.grow_policy
+    * train.max_leaves   
+    * train.max_bin
+    * train.num_parallel_tree    
+ 
+* [Learning Task Params](https://xgboost.readthedocs.io/en/latest/parameter.html#learning-task-parameters)
+    * train.objective
+    * train.eval_metric
+    * train.seed
+    * train.num_round
+        * The number of rounds for boosting
+    * train.num_class
+        * The number of label class in classification task
+        
+* AutoTrain Params
+    * train.convergence_criteria
+        * see supplementary for more details
+    * train.auto_train 
+        * see supplementary for more details
+
+ 
+#### Available pred.attributes
+   * pred.append_columns
+   * pred.prob_column
+   * pred.detail_column
+   * pred.encoding_column
+   
+
+## Overall SQL Syntax for Ant-XGBoost
+### Training Syntax
+```sql
+// standard select clause
+SELECT ... FROM ${TABLE_NAME}
+// train clause
+TRAIN xgboost.${estimatorType}
+WITH
+    [optional] ${train.attributes}
+    ......
+    ......
+COLUMN ${feature_columns}
+[optional] COLUMN ${group_column} FOR group
+[optional] COLUMN ${weight_column} FOR weight
+LABEL ${label_column}
+INTO ${model};
+```
+### Prediction Syntax
+```sql
+// standard select clause
+SELECT ... FROM ${TABLE_NAME}
+// pred clause
+PREDICT ${output_table}.${result_column}
+WITH
+    [optional] ${pred.attributes}
+    ......
+USING ${model};
+```
+
+## Supplementary
+### Generalized Early Stopping Strategy
+`dmlc/xgboost` stops when no significant improvements in the recent n boosting rounds, where n is a configurable parameter.
+In Ant-XGBoost, we generalize this strategy and call the new strategy convergence test. 
+We keep track of the series of metric values and determine whether the series is converged or not. 
+There are three main parameters to test convergence: `minNumPoints`, `n` and `c`. 
+Only after the series is longer than (or equal to)  `minNumPoints`, it start to be eligible for convergence test. 
+Once a series is at least `minNumPoints` long, we find the index `idx` for the best metric value so far. 
+We say a series is converged if `idx + n < size * c`, where `size` is the current number of points in the series. 
+The intuition is that the best metric value should be peaked (or bottomed) with a wide margin. 
+
+With `n` and `c` we can implement complex convergence rules, but there are two common cases.
+* `n > 0` and `c = 1.0`
+  
+   This reduces to the standard early stopping strategy that is employed by dmlc/xgboost.
+
+* `n = 0` and `c in [0, 1]`
+
+    For example, `n = 0` and `c = 0.8`. This means there should be at least 20% of points after the best metric value. Smaller value in `c` leads to a more conservative convergence test. This rule tests convergence in an adaptive way; for some problem the metric values are noisy and grow slowly, this rule will have better chance to find the optimal model. 
+
+In addition, convergence test understands the optimization direction for all built-in metrics, so there is no need to set `maximize` parameter (defaults to `false`, forgetting to set this parameter often leads to strange behavior when metric value should be maximized).
+
+
+### AutoTrain
+With convergence test, we implement a simple `auto_train` method. There are several components in `auto_train`:
+* Automatic parameter validation, setting and rewriting
+
+    Setting the right parameters in XGBoost is not easy. For example, when working with `binary:logistic`, one should not set `num_class` to 2 (otherwise XGBoost will fail with an exception). 
+    
+    In Ant-XGBoost, we validate parameters to make sure all parameters are consistent with each other, e.g., `num_class = 3` and `objective = binary:logistic` will raise an exception. 
+    
+    In addition, we try our best to understand the input parameters from the user and automatically set or rewrite some parameters in `auto_train` mode. 
+    For example, when the feature dimension is very high, building a single tree will be very inefficient,
+    we will automatically set `colsample_bytree` and make sure at most 2000 features are used to build for each tree. Note that automatic parameter rewritting is only turned on in `auto_train` mode, in standard `train`, we only validate parameters and the behavior is fully controled by the user.
+
+* automatic training
+
+    With convergence test, number of trees in a boosted ensemble becomes a less important parameter; 
+    one can always set a very large number and rely on convergence test to figure out the right number of trees. 
+    The most important parameters to tune in boosted trees now become learning rate and max depth. 
+    In Ant-XGBoost, we employ grid search with early stopping to efficiently search the best model structure; 
+    unpromising learning rate or depth will be skipped entirely.
+    
+    While the current `auto_train` method is a very simple approach, we are working on better strategies to further scale up hyperparameter tuning in XGBoost training.