📈 feat: add MemoryObserver for GPU-CPU communication tracking

Implement MemoryObserver to track memory transfers between GPU and CPU:
- H2D (Host to Device): CPU → GPU transfers
- D2H (Device to Host): GPU → CPU transfers
- D2D (Device to Device): GPU buffer copies
- Supports prefill/decode phase separation

Integration points in offload_engine.py:
- load_to_slot_layer: H2D with is_prefill parameter
- offload_slot_layer_to_cpu, offload_prefill_buffer_async: D2H
- write_to_prefill_buffer, write_to_decode_buffer: D2D
- load_block_sample_from_cpu, load_block_full_from_cpu: H2D

Add bench_offload.py integration for memory stats printing.

Benchmark results (Llama-3.1-8B, 64K context):
- Full Policy: Prefill H2D 262.13 GB
- XAttention: Prefill H2D 386.62 GB (1.48x)

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/utils/memory_observer.py

@@ -0,0 +1,133 @@
+"""
+MemoryObserver - 内存传输统计 Observer。
+
+统计 GPU-CPU 间的数据传输量：
+- H2D (Host to Device): CPU → GPU
+- D2H (Device to Host): GPU → CPU
+- D2D (Device to Device): GPU → GPU (buffer copy)
+"""
+
+from nanovllm.utils.observer import Observer
+
+
+class MemoryObserver(Observer):
+    """
+    内存传输 Observer，统计 GPU-CPU 间的数据传输量。
+
+    统计类型：
+    - H2D (Host to Device): CPU → GPU
+    - D2H (Device to Host): GPU → CPU
+    - D2D (Device to Device): GPU → GPU (buffer copy)
+
+    统计位置（均在 offload_engine.py）：
+    - H2D: load_to_slot_layer(), load_block_sample_from_cpu(), load_block_full_from_cpu()
+    - D2H: offload_slot_layer_to_cpu(), offload_prefill_buffer_async()
+    - D2D: write_to_prefill_buffer(), write_to_decode_buffer()
+    - 重置: llm_engine.py:generate() - 与 InferenceObserver 一起重置
+    """
+
+    _enabled: bool = False  # 默认禁用，需要显式启用
+
+    # H2D 统计
+    h2d_bytes: int = 0
+    h2d_count: int = 0
+
+    # D2H 统计
+    d2h_bytes: int = 0
+    d2h_count: int = 0
+
+    # D2D 统计
+    d2d_bytes: int = 0
+    d2d_count: int = 0
+
+    # 按阶段统计
+    prefill_h2d_bytes: int = 0
+    prefill_d2h_bytes: int = 0
+    decode_h2d_bytes: int = 0
+    decode_d2h_bytes: int = 0
+
+    @classmethod
+    def record_h2d(cls, num_bytes: int, is_prefill: bool = True) -> None:
+        """记录 H2D 传输"""
+        if not cls._enabled:
+            return
+        cls.h2d_bytes += num_bytes
+        cls.h2d_count += 1
+        if is_prefill:
+            cls.prefill_h2d_bytes += num_bytes
+        else:
+            cls.decode_h2d_bytes += num_bytes
+
+    @classmethod
+    def record_d2h(cls, num_bytes: int, is_prefill: bool = True) -> None:
+        """记录 D2H 传输"""
+        if not cls._enabled:
+            return
+        cls.d2h_bytes += num_bytes
+        cls.d2h_count += 1
+        if is_prefill:
+            cls.prefill_d2h_bytes += num_bytes
+        else:
+            cls.decode_d2h_bytes += num_bytes
+
+    @classmethod
+    def record_d2d(cls, num_bytes: int) -> None:
+        """记录 D2D 传输"""
+        if not cls._enabled:
+            return
+        cls.d2d_bytes += num_bytes
+        cls.d2d_count += 1
+
+    @classmethod
+    def complete_reset(cls) -> None:
+        """重置所有统计"""
+        cls.h2d_bytes = cls.h2d_count = 0
+        cls.d2h_bytes = cls.d2h_count = 0
+        cls.d2d_bytes = cls.d2d_count = 0
+        cls.prefill_h2d_bytes = cls.prefill_d2h_bytes = 0
+        cls.decode_h2d_bytes = cls.decode_d2h_bytes = 0
+
+    @classmethod
+    def get_summary(cls) -> dict:
+        """返回统计摘要"""
+        return {
+            "total": {
+                "h2d_bytes": cls.h2d_bytes,
+                "h2d_count": cls.h2d_count,
+                "d2h_bytes": cls.d2h_bytes,
+                "d2h_count": cls.d2h_count,
+                "d2d_bytes": cls.d2d_bytes,
+                "d2d_count": cls.d2d_count,
+            },
+            "prefill": {
+                "h2d_bytes": cls.prefill_h2d_bytes,
+                "d2h_bytes": cls.prefill_d2h_bytes,
+            },
+            "decode": {
+                "h2d_bytes": cls.decode_h2d_bytes,
+                "d2h_bytes": cls.decode_d2h_bytes,
+            },
+        }
+
+    @classmethod
+    def _fmt_bytes(cls, b: int) -> str:
+        """格式化字节数"""
+        if b >= 1e9:
+            return f"{b/1e9:.2f} GB"
+        if b >= 1e6:
+            return f"{b/1e6:.2f} MB"
+        if b >= 1e3:
+            return f"{b/1e3:.2f} KB"
+        return f"{b} B"
+
+    @classmethod
+    def print_summary(cls) -> None:
+        """打印人类可读的摘要"""
+        fmt = cls._fmt_bytes
+        total = cls.h2d_bytes + cls.d2h_bytes + cls.d2d_bytes
+        print(f"[MemoryObserver] Total: {fmt(total)}")
+        print(f"  H2D: {fmt(cls.h2d_bytes)} ({cls.h2d_count} ops)")
+        print(f"  D2H: {fmt(cls.d2h_bytes)} ({cls.d2h_count} ops)")
+        print(f"  D2D: {fmt(cls.d2d_bytes)} ({cls.d2d_count} ops)")
+        print(f"  Prefill - H2D: {fmt(cls.prefill_h2d_bytes)}, D2H: {fmt(cls.prefill_d2h_bytes)}")
+        print(f"  Decode  - H2D: {fmt(cls.decode_h2d_bytes)}, D2H: {fmt(cls.decode_d2h_bytes)}")