[Bugfix][ROCm] Strip block_size before attention backend validation (#36274)

Signed-off-by: jennyyyyzhen <yzhen@hmc.edu>
Co-authored-by: Lu Fang <30275821+houseroad@users.noreply.github.com>

vllm/platforms/rocm.py

@@ -438,6 +438,8 @@ class RocmPlatform(Platform):
         device_capability = cls.get_device_capability()
         assert device_capability is not None
 
+        attn_selector_config = attn_selector_config._replace(block_size=None)
+
         # First try checking just the selected backend, if there is one.
         if selected_backend is not None:
             try: