fix(thunder): remove duplicate _configure_launcher that dropped the external-launch guard

[Anai-Guo]

Problem

ThunderDDPStrategy in extensions/thunder/strategies/thunder_ddp.py defines _configure_launcher twice within the same class:

@override
def _configure_launcher(self) -> None:
    assert self.cluster_environment is not None
    if not self.cluster_environment.creates_processes_externally:   # guarded
        self._launcher = _SubprocessScriptLauncher(self.cluster_environment, self.num_processes, self.num_nodes)

@property
def process_group_backend(self) -> str | None:
    return self._process_group_backend

@override
def _configure_launcher(self) -> None:                              # duplicate, wins
    assert self.cluster_environment is not None
    self._launcher = _SubprocessScriptLauncher(self.cluster_environment, self.num_processes, self.num_nodes)

The second definition silently overrides the first and drops the if not self.cluster_environment.creates_processes_externally guard. As a result, a _SubprocessScriptLauncher is always installed — even when the processes are launched externally (e.g. torchrun, SLURM, or any environment where creates_processes_externally is True). In that case the strategy should not create a subprocess launcher, otherwise it attempts to re-spawn the worker processes itself.

Fix

Remove the unguarded duplicate, keeping the guarded version. This matches:

• the sibling ThunderFSDPStrategy._configure_launcher in the same directory (thunder_fsdp.py), which has the single, guarded implementation, and
• Lightning's own DDPStrategy._configure_launcher.

No behavior change when launching normally; the guard is now honored again for externally-launched processes.

🤖 Generated with Claude Code

[bhimrazy]

LGTM, Thanks @Anai-Guo