[ROCm] Fix KV copy methods and auto-select attention backend for ROCm (#36845)

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

vllm/platforms/rocm.py

@@ -851,6 +851,30 @@ class RocmPlatform(Platform):
                     "`dtype` flag in CLI, for example: --dtype=half."
                 )
 
+    @classmethod
+    def insert_blocks_to_device(
+        cls,
+        src_cache: torch.Tensor,
+        dst_cache: torch.Tensor,
+        src_block_indices: torch.Tensor,
+        dst_block_indices: torch.Tensor,
+    ) -> None:
+        """Copy blocks from src_cache to dst_cache on GPU."""
+        _src_cache = src_cache[:, src_block_indices]
+        dst_cache[:, dst_block_indices] = _src_cache.to(dst_cache.device)
+
+    @classmethod
+    def swap_out_blocks_to_host(
+        cls,
+        src_cache: torch.Tensor,
+        dst_cache: torch.Tensor,
+        src_block_indices: torch.Tensor,
+        dst_block_indices: torch.Tensor,
+    ) -> None:
+        """Copy blocks from GPU to host (CPU)."""
+        _src_cache = src_cache[:, src_block_indices]
+        dst_cache[:, dst_block_indices] = _src_cache.cpu()
+
     @classmethod
     def support_hybrid_kv_cache(cls) -> bool:
         return True