[Kernel] Raise verbose error and consolidate num_heads/num_kv_heads divisibility check (#19339)

Signed-off-by: 22quinn <33176974+22quinn@users.noreply.github.com>

tests/kernels/attention/test_attention.py

@@ -10,6 +10,7 @@ import torch
 from tests.kernels.allclose_default import get_default_atol, get_default_rtol
 from tests.kernels.utils import opcheck
 from vllm import _custom_ops as ops
+from vllm.attention.layer import Attention, MultiHeadAttention
 from vllm.platforms import current_platform
 from vllm.utils import get_max_shared_memory_bytes
 
@@ -506,3 +507,18 @@ def test_multi_query_kv_attention_with_alibi(
         device,
         use_alibi=True,
     )
+
+
+@pytest.mark.parametrize("attention_cls", [Attention, MultiHeadAttention])
+def test_num_heads_not_divisble_by_num_kv_heads(attention_cls: type) -> None:
+    head_size = 64
+    scale = float(1.0 / (head_size**0.5))
+    num_heads = 16
+    num_kv_heads = 5
+    with pytest.raises(AssertionError):
+        _ = attention_cls(
+            num_heads=num_heads,
+            head_size=head_size,
+            scale=scale,
+            num_kv_heads=num_kv_heads,
+        )