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
• https://github.com/pytorch/torchx/blob/420921847296790e8bfa5cd43c8d9776a3f88165/torchx/specs/finder.py#L322

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For your consideration, @d4l3k @kiukchung @tonykao8080

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Any concerns before I submit the contribution, @d4l3k , @kiukchung , @tonykao8080?

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The fix to simply merge/overwrite existing schedulers is trivial (publishing PR). The only question is whether we want to support the same _0/_1 behavior as for components here