New sqlflow models may not pass attribute validation

[typhoonzero]

If we develop a new Keras model under the models repo and try using it in SQLFlow, if the model arguments are not listed in model_parameters.go, the train SQL will report errors like:

---------------------------------------------------------------------------
_MultiThreadedRendezvous                  Traceback (most recent call last)
<ipython-input-3-cae0cd58c2ab> in <module>
----> 1 get_ipython().run_cell_magic('sqlflow', '', 'SELECT * FROM iris.train \nTO TRAIN sqflow_models.LSTMBasedTimeSeriesModel \nWITH\n  model.n_in = 4,\n  model.stack_units=[50, 50],\n  model.n_out = 1\nLABEL class\nINTO sqlflow_models.my_lstmts_model;\n')

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/IPython/core/interactiveshell.py in run_cell_magic(self, magic_name, line, cell)
   2350             with self.builtin_trap:
   2351                 args = (magic_arg_s, cell)
-> 2352                 result = fn(*args, **kwargs)
   2353             return result
   2354 

</miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/decorator.py:decorator-gen-126> in execute(self, line, cell)

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/IPython/core/magic.py in <lambda>(f, *a, **k)
    185     # but it's overkill for just that one bit of state.
    186     def magic_deco(arg):
--> 187         call = lambda f, *a, **k: f(*a, **k)
    188 
    189         if callable(arg):

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/sqlflow/magic.py in execute(self, line, cell)
     43 
     44         """
---> 45         return self.client.execute('\n'.join([line, cell]))
     46 
     47 def load_ipython_extension(ipython):

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/sqlflow/client.py in execute(self, operation)
    111             # NOTE: raise exception to interrupt notebook execution. Or
    112             # the notebook will continue execute the next block.
--> 113             raise e
    114         except EnvExpanderError as e:
    115             raise e

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/sqlflow/client.py in execute(self, operation)
    107         try:
    108             stream_response = self._stub.Run(self.sql_request(operation), timeout=DEFAULT_TIMEOUT)
--> 109             return self.display(stream_response)
    110         except grpc.RpcError as e:
    111             # NOTE: raise exception to interrupt notebook execution. Or

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/sqlflow/client.py in display(cls, stream_response)
    121         while True:
    122             try:
--> 123                 first = next(stream_response)
    124             except StopIteration:
    125                 break

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/grpc/_channel.py in __next__(self)
    414 
    415     def __next__(self):
--> 416         return self._next()
    417 
    418     def _next(self):

/miniconda/envs/sqlflow-dev/lib/python3.6/site-packages/grpc/_channel.py in _next(self)
    701                     raise StopIteration()
    702                 elif self._state.code is not None:
--> 703                     raise self
    704 
    705 

_MultiThreadedRendezvous: <_MultiThreadedRendezvous of RPC that terminated with:
  status = StatusCode.UNKNOWN
  details = "unsupported attribute model.n_in"
  debug_error_string = "{"created":"@1577434767.807456700","description":"Error received from peer ipv4:127.0.0.1:50051","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"unsupported attribute model.n_in","grpc_status":2}"
>