[torchx.pipelines] Add an Airflow pipeline adapter

[kiukchung]

Description

Add support for airflow pipeline adapter for torchx AppDefs.

Motivation/Background

torchx-0.1.0.dev only supports Kubeflow Pipelines

Detailed Proposal

Essentially add: https://github.com/pytorch/torchx/tree/master/torchx/pipelines/kfp for airflow

Alternatives

N/A

Additional context/links

N/A

[d4l3k]

There's been at least one user asking about this: https://pytorch.slack.com/archives/C2077MFDL/p1634845465005300?thread_ts=1634842494.004200&cid=C2077MFDL

[d4l3k]

Example at https://pytorch.org/torchx/main/pipelines/airflow.html

If we think we need a full airflow component / or someone wants to contribute one we can reopen this down the line