CI: Define reusable workflows and composite actions

[akihironitta]

Proposed refactor

Some of the steps/workflows are repetitive across workflows/repositories, which makes it very time-consuming to make sure that all of our workflow files are updated in the same way.

Items to be decoupled

• caching-related steps
• downloading legacy checkpoints
• dependency installation
• ... (open for suggestions!)

Motivation

To improve maintainability.

(I think we should first focus only on this repository, but it will also beneficial for other repositories in the future as these decoupled actions/workflows can be called from other repositories.)

Pitch

1. Decouple repetitive steps from existing workflows, and pack them into composite actions in the following directory:

.github/actions/*/config.yml

2. Decouple repetitive workflows from our repositories and define reusable workflows (reusable workflows are different from composite actions above).

.github/workflows/_*.yml

Additional context

• Planning to utilise:
  • reusable workflows: https://docs.github.com/en/actions/using-workflows/reusing-workflows#reusable-workflows-and-starter-workflows
  • custom actions: https://docs.github.com/en/actions/creating-actions
• Similar to torchmetrics CI: https://github.com/PyTorchLightning/metrics/tree/master/.github/actions

---

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cc @carmocca @akihironitta @Borda @justusschock @awaelchli @rohitgr7

[Borda]

you can check PoC from Metrics: https://github.com/PyTorchLightning/metrics/tree/master/.github/actions/caching