List all registered torchx.schedulers

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Description

Currently we can only have the default ones or the custom ones (from one of the packages): https://github.com/pytorch/torchx/blob/main/torchx/schedulers/__init__.py#L59

Motivation/Background

This will allow adding new schedulers locally without re-registering the default schedulers. The list gets stale when new default schedulers are added to TorchX

Detailed Proposal

Make it work the same way as named_resources (simply list all) or components (allow referencing via _0, _1, etc.)

Alternatives

Keep re-registering the default schedulers

Additional context/links

• https://github.com/pytorch/torchx/blob/main/torchx/schedulers/__init__.py#L59
• https://github.com/pytorch/torchx/blob/main/torchx/specs/__init__.py#L65

• torchx/torchx/specs/finder.py

  Line 322 in 4209218

  if not components:

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For your consideration, @d4l3k @kiukchung @tonykao8080