configs/base: remove override /cluster from experiment defaults

The /cluster default group no longer exists (cluster.py was dropped and the
cluster entry removed from configs/base/config.py's defaults list). Hydra
errors with ConfigCompositionException when an experiment tries to override
a missing group, so strip the override from the three remaining experiments:
vision_sft_nano, vision_sft_super, action_policy_droid_nano.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yy-code-nv

cosmos_framework/configs/base/experiment/action/posttrain_config/action_policy_droid_nano.py

@@ -59,7 +59,6 @@
             {"override /ema": "power"},
             {"override /tokenizer": "wan2pt2_tokenizer"},
             {"override /sound_tokenizer": None},
-            {"override /cluster": None},
             {"override /vlm_config": None},
             {"override /ckpt_type": "dcp"},
             "_self_",

cosmos_framework/configs/base/experiment/sft/vision_sft_nano.py

@@ -78,7 +78,6 @@
             {"override /ema": "power"},
             {"override /tokenizer": "wan2pt2_tokenizer"},
             {"override /sound_tokenizer": None},
-            {"override /cluster": None},
             {"override /vlm_config": None},
             {"override /ckpt_type": "dcp"},
             "_self_",

cosmos_framework/configs/base/experiment/sft/vision_sft_super.py

@@ -87,7 +87,6 @@
             {"override /ema": "power"},
             {"override /tokenizer": "wan2pt2_tokenizer"},
             {"override /sound_tokenizer": None},
-            {"override /cluster": None},
             {"override /vlm_config": None},
             {"override /ckpt_type": "dcp"},
             "_self_",