[ROCm][CI] Guard CudaPlatform/RocmPlatform imports to fix test collection on cross-platform builds (#37617)

Signed-off-by: Andreas Karatzas <akaratza@amd.com>

tests/kernels/attention/test_attention_selector.py

@@ -14,8 +14,19 @@ from vllm.config import (
 )
 from vllm.platforms import current_platform
 from vllm.platforms.cpu import CpuPlatform
-from vllm.platforms.cuda import CudaPlatform
-from vllm.platforms.rocm import RocmPlatform
+
+# CudaPlatform and RocmPlatform import their respective compiled C extensions
+# at module level, raising ModuleNotFoundError on incompatible builds.
+try:
+    from vllm.platforms.cuda import CudaPlatform
+except (ImportError, ModuleNotFoundError):
+    CudaPlatform = None
+
+try:
+    from vllm.platforms.rocm import RocmPlatform
+except (ImportError, ModuleNotFoundError):
+    RocmPlatform = None
+
 from vllm.v1.attention.backends.registry import AttentionBackendEnum
 from vllm.v1.attention.selector import _cached_get_attn_backend, get_attn_backend
 
@@ -101,6 +112,8 @@ def test_backend_selection(
             assert backend.get_name() == "CPU_ATTN"
 
         elif device == "hip":
+            if RocmPlatform is None:
+                pytest.skip("RocmPlatform not available")
             with patch("vllm.platforms.current_platform", RocmPlatform()):
                 if use_mla:
                     # ROCm MLA backend logic:
@@ -126,6 +139,8 @@ def test_backend_selection(
                     assert backend.get_name() == expected
 
         elif device == "cuda":
+            if CudaPlatform is None:
+                pytest.skip("CudaPlatform not available")
             with patch("vllm.platforms.current_platform", CudaPlatform()):
                 capability = torch.cuda.get_device_capability()
                 if use_mla:
@@ -214,7 +229,7 @@ def test_backend_selection(
                     assert backend.get_name() == expected
 
 
-@pytest.mark.parametrize("device", ["cpu", "cuda"])
+@pytest.mark.parametrize("device", ["cpu", "cuda", "hip"])
 def test_fp32_fallback(device: str):
     """Test attention backend selection with fp32."""
     # Use default config (no backend specified)
@@ -227,10 +242,25 @@ def test_fp32_fallback(device: str):
             assert backend.get_name() == "CPU_ATTN"
 
         elif device == "cuda":
+            if CudaPlatform is None:
+                pytest.skip("CudaPlatform not available")
             with patch("vllm.platforms.current_platform", CudaPlatform()):
                 backend = get_attn_backend(16, torch.float32, None)
             assert backend.get_name() == "FLEX_ATTENTION"
 
+        elif device == "hip":
+            if RocmPlatform is None:
+                pytest.skip("RocmPlatform not available")
+            # ROCm backends do not support head_size=16 (minimum is 32).
+            # No known HuggingFace transformer model uses head_size=16.
+            # Revisit if a real model with this head size is identified
+            # and accuracy-tested.
+            with (
+                patch("vllm.platforms.current_platform", RocmPlatform()),
+                pytest.raises(ValueError, match="No valid attention backend"),
+            ):
+                get_attn_backend(16, torch.float32, None)
+
 
 def test_flash_attn(monkeypatch: pytest.MonkeyPatch):
     """Test FlashAttn validation."""
@@ -367,6 +397,8 @@ def test_per_head_quant_scales_backend_selection(
         attention_config=attention_config, cache_config=cache_config
     )
 
+    if CudaPlatform is None:
+        pytest.skip("CudaPlatform not available")
     with (
         set_current_vllm_config(vllm_config),
         patch("vllm.platforms.current_platform", CudaPlatform()),