Fix placement group crash for external engines under debug_rollout_only - #2170
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With --rollout-external-engine-addrs + --debug-rollout-only, rollout_num_gpus gets overwritten by the remote engines' GPU count (overriding local actor dims), yet _get_placement_group_layout returns (0, 0) for this combo, the empty placement group mismatches the non-zero actor world_size and crashes with IndexError here
slime/slime/ray/actor_group.py
Line 114 in 2346470
Skip the actor-dim override when rollout_external in the debug_rollout_only branch, and return (actor_num_gpus, 0) from _get_placement_group_layout so the PG fits the placeholder training actors.
Note: this fix assumes the node has GPUs available to create the placeholder training actors.
Also updated the unit test, since the previous
(0, 0)expectation would lead to the crash.