[Torchelastic][Logging] Pluggable logsspecs using python entrypoints and option to specify one by name. (#120942)
Summary:
Expose an option to users to specify name of the LogsSpec implementation to use.
- Has to be defined in entrypoints under `torchrun.logs_specs` group.
- Must implement LogsSpec defined in prior PR/diff.
Test Plan: unit test+local tests
Reviewed By: ezyang
Differential Revision: D54180838
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120942
Approved by: https://github.com/ezyang
