Created by: qingqing01

1. 更改目录结构：

|-- examples
|-- hapi
|   `-- tests

2. 增加Fit接口 @xiangyang 讨论，觉得callback复杂，提了一个无callback，在接口设置打印log和保存模型的接口，只能处理最简单的场景。复杂场景没有callback无法处理。

    def fit(
            self,
            train_iterator=None,
            eval_iterator=None,
            eval_freq=1,
            epochs=1,
            device=None,
            log_step=10,
            save_freq=1,
            save_filepath='output',
            verbose=2, ):

默认打印Log:

Epoch 0/1
step     0 - loss: 2.2957 - 0s/step - elapsed 4s
step    10 - loss: 0.6539 - 437ms/step - elapsed 4s
step    20 - loss: 0.4263 - 223ms/step - elapsed 4s
step    30 - loss: 0.2812 - 152ms/step - elapsed 5s
step    40 - loss: 0.2512 - 116ms/step - elapsed 5s
step    50 - loss: 0.2515 - 95ms/step - elapsed 5s
...