Flink-ML: Sink model data in online training

[thekingofcity]

Hi,

I want sink the model data (coefficient from the logsitic regression model in 
my case) from the flink.ml.api.Model to print or file. I figure out the way to 
sink it in the batch training mode but face the following exception when the 
Estimator takes an UNBOUNDED datastream.

```

Caused by: java.lang.IllegalStateException: There can be only a single consumer 
in a FeedbackChannel.
at 
org.apache.flink.statefun.flink.core.feedback.FeedbackChannel.registerConsumer(FeedbackChannel.java:79)
```

This will happend if I dump it through the Table API like this:

```
final TableDescriptor sinkDescriptor = TableDescriptor
.forConnector("print")
.schema(Schema
.newBuilder()
.column("coefficient", DataTypes.of(new DenseVectorTypeInfo()))
.build()
).build();
tEnv.createTemporaryTable("ModelSink", sinkDescriptor);
model.getModelData()[0].executeInsert("ModelSink");
```

Looking for an example that can sink the model data in online training mode.

With many thanks,
thekingofcity