[Model Zoo] TODO list of implementing model zoo

[typhoonzero]

Design doc: https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/design/model_zoo.md

1. unify SQLFlow and ElasticDL custom models Unify model zoo implementation of SQLFlow and ElasticDL models#22.
2. support keras functional API models: Support Keras functional API in SQLFlow #1093
3. support parse model name like a_data_scientist/regressors:v0.2/MyDNNRegressor
4. support pytorch custom model in model zoo.
5. base Docker image for building all model Docker images, with python, ElasticDL, and other common dependencies.
6. save model weights in a database table with columns of metadatas. (save in MySQL, Hive, MaxCompute)
7. HTTP service models.sqlflow.org to serve publishing and listing models.
8. submitter program runs a model in a docker image.
9. support using models.sqlflow.org in SQL statements.
10. support SQLFLOW LIST|PUBLISH command.

[typhoonzero]

Related with couler: #1066

[typhoonzero]

Model zoo is implemented in latest SQLFlow.