[CI] Shard four AMD mirrors automatically (#61817)

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

.buildkite/test_areas/entrypoints.yaml

@@ -254,6 +254,7 @@ steps:
     amd:
       label: ":amd: (MI355 DPX) Entrypoints Integration (Pooling)"
       dind: false
+      automatic_shard: true
       device: mi355_dpx
       timeout_in_minutes: 65
       working_dir: "/vllm-workspace/tests"

.buildkite/test_areas/kernels.yaml

@@ -532,6 +532,7 @@ steps:
     amd:
       label: ":amd: (MI250) Mamba Kernels"
       dind: false
+      automatic_shard: true
       device: mi250_1
       timeout_in_minutes: 55
       depends_on:

.buildkite/test_areas/quantization.yaml

@@ -137,6 +137,7 @@ steps:
     amd:
       label: ":amd: (MI355 DPX) Quantized Models"
       dind: false
+      automatic_shard: true
       device: mi355_dpx
       timeout_in_minutes: 80
       working_dir: "/vllm-workspace/tests"

.buildkite/test_areas/spec_decode.yaml

@@ -28,6 +28,7 @@ steps:
     amd:
       label: ":amd: (MI300) V1 Spec Decode"
       dind: false
+      automatic_shard: true
       device: mi300_1
       timeout_in_minutes: 50
       depends_on: