✨ feat: integrate sparse policy architecture into GPU-only mode

- Add compute_prefill() and compute_decode() GPU-only methods to SparsePolicy base class
- Implement GPU-only methods in FullAttentionPolicy using flash_attn
- Add sparse_policy parameter to GPUOnlyManager
- Update create_kvcache_manager() to create FullAttentionPolicy for GPU-only mode
- Route GPU-only attention through sparse_policy in attention.py
- Pass kvcache_manager to context for policy access
- Add --enable-policy flag to bench.py for testing
- Handle warmup phase when kvcache_manager is not yet allocated

This allows GPU-only mode to use the same policy architecture as CPU offload mode,
enabling future sparse attention implementations (Quest, XAttention) in GPU-only mode.

Performance verified: ~4890 tok/s (unchanged from baseline)

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

nanovllm/kvcache/sparse/policy.py

@@ -191,6 +191,87 @@ class SparsePolicy(ABC):
         """
         pass
 
+    # =========================================================================
+    # GPU-only methods (non-chunked)
+    # These methods are used when all KV cache is on GPU, no CPU offload needed.
+    # =========================================================================
+
+    def compute_prefill(
+        self,
+        q: torch.Tensor,
+        k: torch.Tensor,
+        v: torch.Tensor,
+        cu_seqlens_q: torch.Tensor,
+        cu_seqlens_k: torch.Tensor,
+        max_seqlen_q: int,
+        max_seqlen_k: int,
+        softmax_scale: float,
+        layer_id: int,
+        block_tables: Optional[torch.Tensor] = None,
+    ) -> torch.Tensor:
+        """
+        Compute GPU-only prefill attention (non-chunked).
+
+        This method is used when all KV cache resides on GPU (no CPU offload).
+        Override this to implement sparse prefill attention for GPU-only mode.
+        Default implementation raises NotImplementedError.
+
+        Args:
+            q: [total_q, num_heads, head_dim] query tensor (packed variable length)
+            k: [total_kv, num_kv_heads, head_dim] key tensor
+            v: [total_kv, num_kv_heads, head_dim] value tensor
+            cu_seqlens_q: [batch+1] cumulative sequence lengths for queries
+            cu_seqlens_k: [batch+1] cumulative sequence lengths for keys
+            max_seqlen_q: maximum query sequence length
+            max_seqlen_k: maximum key sequence length
+            softmax_scale: softmax scaling factor (1/sqrt(head_dim))
+            layer_id: transformer layer index
+            block_tables: [batch, max_blocks] paged attention block tables (optional)
+
+        Returns:
+            [total_q, num_heads, head_dim] attention output
+        """
+        raise NotImplementedError(
+            f"{self.__class__.__name__} does not implement compute_prefill for GPU-only mode"
+        )
+
+    def compute_decode(
+        self,
+        q: torch.Tensor,
+        k_cache: torch.Tensor,
+        v_cache: torch.Tensor,
+        cache_seqlens: torch.Tensor,
+        softmax_scale: float,
+        layer_id: int,
+        block_tables: Optional[torch.Tensor] = None,
+    ) -> torch.Tensor:
+        """
+        Compute GPU-only decode attention (non-chunked).
+
+        This method is used when all KV cache resides on GPU (no CPU offload).
+        Override this to implement sparse decode attention for GPU-only mode.
+        Default implementation raises NotImplementedError.
+
+        Args:
+            q: [batch, num_heads, head_dim] query tensor (single token per sequence)
+            k_cache: [num_blocks, block_size, num_kv_heads, head_dim] paged key cache
+            v_cache: [num_blocks, block_size, num_kv_heads, head_dim] paged value cache
+            cache_seqlens: [batch] sequence lengths in cache
+            softmax_scale: softmax scaling factor (1/sqrt(head_dim))
+            layer_id: transformer layer index
+            block_tables: [batch, max_blocks] paged attention block tables (optional)
+
+        Returns:
+            [batch, 1, num_heads, head_dim] attention output
+        """
+        raise NotImplementedError(
+            f"{self.__class__.__name__} does not implement compute_decode for GPU-only mode"
+        )
+
+    # =========================================================================
+    # Chunked offload methods (for CPU offload mode)
+    # =========================================================================
+
     @abstractmethod
     def compute_chunked_prefill(
         self,