Fix a regression introduced by overlapping KV cache writing (#4375)

python/sglang/srt/mem_cache/memory_pool.py

@@ -326,7 +326,7 @@ class MHATokenToKVPool(KVCache):
             cache_k = cache_k.view(self.store_dtype)
             cache_v = cache_v.view(self.store_dtype)
 
-        if self.capture_mode:
+        if self.capture_mode and cache_k.shape[0] < 4:
             self.alt_stream.wait_stream(torch.cuda.current_stream())
             with torch.cuda.stream(self.alt_stream):
                 self.k_buffer[layer_id][loc] = cache_k