Add support for Keras Models SQL generation

[antoinecarme]

Allow generation deployment SQL for keras models using this web service.

https://github.com/antoinecarme/keras2sql

Deliverables :

1. Include keras models,
2. SQL generation and
3. the necessary tests,
4. sample python/keras script

[antoinecarme]

Need to :

1. serialize the trained keras model (json + hdf or using pickle, not easy)
2. send the serialized model through the web service (easy).
3. read back the request result and extract the SQL code.

[antoinecarme]

Added some issue for making keras models picklable :

keras-team/keras#10475

Some serious effort already started :

keras-team/keras#10483

[antoinecarme]

As of 2021-05-04 , Keras models do not pickle properly, the test script

https://github.com/antoinecarme/keras2sql/blob/master/test_client.py

which builds a simple RNN classification model on the iris dataset fails to pickle with the following error

Traceback (most recent call last):
  File "/home/antoine/dev/python/packages/sql_deploy/keras2sql/test_client.py", line 64, in <module>
    pickle_data = pickle.dumps(clf)
TypeError: cannot pickle 'weakref' object

Keep the faith that such an issue be solved someday.

keras-team/keras#10475