Support chain dag in example_app

[Michaelvll]

This PR supports the chain dag in the example_app.py. It is now available to run the chain dag.

Tested:

• examples/example_app.py. It now can handle the input/output automatically.
• Mocked training and inference with Inferentia.

Note: This is experimental support for chain dag. Need a lot of refactorization.

Future TODO:

• Real training, inference pipeline.

• GCS file mounts Cloud storage file mounts for gs:// #269

• Multiple input/outputs

• Do not download the cloud storage on remote

  • Proposed API:

  # train.py will read from the CLOUD storage (CLOUD may be decided by the optimizer) directly
  task.run = "python train.py --input SKY_INPUTS[0]"
  task.set_inputs('CLOUD://sky-example-test', estimated_size_gigabytes=150)
  
  # train.py will read from local disk
  task.run = "python train.py --input local_path
  task.set_inputs('CLOUD://sky-example-test', estimated_size_gigabytes=150)
  task.set_filemounts({local_path: 'SKY_INPUTS[0]'})