Generate the transform code from the SQLFlow statement using code_gen

[brightcoder01]

The root of the discussion series is #1670
The SQLFlow syntax design of data transform is discussed in #1664

From this SQLFlow statement, we will then generate the transform python code.
There are three options for the style of the generated transform code:

1. Feature Column API. Integrate it with model definition using tf.keras.layers.DenseFeatures;
2. Customized Keras Layer provided from ElasticDL. The functionality should cover all the common used feature engineering operations above;
3. Keras Preprocess Layer. This will be ready in TF2.2;

[brightcoder01]

COLUMN expression -> Parser -> intermediate representative (Transform DAG, Table Schema) -> Generated Python Code.

The parsing result for the COLUMN expression is a DAG, we can see the clause in this sample