AutoML pipelineData metrics

@harneetvirk @swinner95 @vizhur @rastala @amlrelsa-ms

In a new AutoMLStep class, we have to give metrics data as pipeline data for input. However how to access this metric data in another step..

```
metrics_data = PipelineData(name='metrics_data',
datastore=ds,
pipeline_output_name=metrics_output_name,
training_output=TrainingOutput(type='Metrics'))
```

https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/machine-learning-pipelines/intro-to-pipelines/aml-pipelines-with-automated-machine-learning-step.ipynb

After the automl step is run and metrics_data is written somewhere, i want to read the metrics data file(json) in another step. I found a way to download this metrics_data using pipelinerun object but couldnt find a way how to read the metrics_data file in next step directly..