[CI] Shard the AMD mirrors of the DP, RayExecutorV2 and batch invariance steps automatically (#61861)

Signed-off-by: Thang Nguyen <thangnguyenvn647@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.buildkite/test_areas/distributed.yaml

@@ -109,6 +109,7 @@ steps:
     amd:
       label: ":amd: (MI355) Distributed DP Basic"
       dind: false
+      automatic_shard: true
       device: mi355_2
       timeout_in_minutes: 45
       depends_on:
@@ -331,6 +332,7 @@ steps:
     amd:
       label: ':amd: (MI355) Distributed DP Extended'
       dind: false
+      automatic_shard: true
       device: mi355_4
       timeout_in_minutes: 60
       working_dir: /vllm-workspace/tests
@@ -706,6 +708,7 @@ steps:
     amd:
       label: ':amd: (MI355) RayExecutorV2'
       dind: false
+      automatic_shard: true
       device: mi355_4
       timeout_in_minutes: 75
       working_dir: /vllm-workspace/tests

.buildkite/test_areas/misc.yaml

@@ -709,6 +709,7 @@ steps:
     amd:
       label: ":amd: (MI300) Batch Invariance"
       dind: false
+      automatic_shard: true
       device: mi300_1
       timeout_in_minutes: 60
       depends_on: