[CI] split tests to reduce CI time (vllm-project#48219)

Signed-off-by: zjy0516 <riverclouds.zhu@qq.com>

Author: ZJY0516

.buildkite/test_areas/models_basic.yaml

@@ -27,7 +27,7 @@ steps:
     # subset of supported models (the complement of the small subset in the above
     # test.) Also run if model initialization test file is modified
     - pytest -v -s models/test_initialization.py -k 'not test_can_initialize_small_subset' --num-shards=$$BUILDKITE_PARALLEL_JOB_COUNT --shard-id=$$BUILDKITE_PARALLEL_JOB
-  parallelism: 2
+  parallelism: 4
 
 - label: Basic Models Tests (Other)
   device: h200_35gb

.buildkite/test_areas/models_multimodal.yaml

@@ -69,7 +69,7 @@ steps:
       depends_on:
       - image-build-amd
 
-- label: Multi-Modal Processor (CPU)
+- label: Multi-Modal Processor (CPU) %N
   key: multi-modal-processor-cpu
   depends_on:
   - image-build-cpu
@@ -80,7 +80,8 @@ steps:
   - tests/models/registry.py
   device: cpu-medium
   commands:
-    - pytest -v -s models/multimodal/processing --ignore models/multimodal/processing/test_tensor_schema.py
+    - pytest -v -s models/multimodal/processing --ignore models/multimodal/processing/test_tensor_schema.py --num-shards=$$BUILDKITE_PARALLEL_JOB_COUNT --shard-id=$$BUILDKITE_PARALLEL_JOB
+  parallelism: 4
 
 - label: Multi-Modal Processor # 44min
   key: multi-modal-processor