object of type TrainingInput is not JSON serializable

```
workflow = InferencePipeline(
preprocessor=...,
estimator=...,
inputs={
"train":TrainingInput(s3_data="..."),
"test":TrainingInput(s3_data="...")
},
...
)
```

When you call `execute()` it will fail, saying `object of type TrainingInput is not JSON serializable`

Reproduce the failure with the InferencePipeline example in the issue and call execute(). Trace the serialization path for the inputs containing TrainingInput objects; done means the pipeline executes without the JSON serialization error and both train and test inputs are handled correctly.

Written by the indexing model from the issue text.