Fix torch.compile cacheing (sgl-project#5259)

Co-authored-by: zhyncs <me@zhyncs.com>

Author: zou3519

python/sglang/srt/model_executor/model_runner.py

@@ -64,7 +64,10 @@
 )
 from sglang.srt.model_loader.utils import set_default_torch_dtype
 from sglang.srt.model_loader.weight_utils import default_weight_loader
-from sglang.srt.patch_torch import monkey_patch_torch_reductions
+from sglang.srt.patch_torch import (
+    monkey_patch_torch_compile,
+    monkey_patch_torch_reductions,
+)
 from sglang.srt.sampling.sampling_batch_info import SamplingBatchInfo
 from sglang.srt.server_args import ServerArgs
 from sglang.srt.speculative.spec_info import SpeculativeAlgorithm
@@ -88,6 +91,8 @@
 SGLANG_CI_SMALL_KV_SIZE = os.getenv("SGLANG_CI_SMALL_KV_SIZE", None)
 UNBALANCED_MODEL_LOADING_TIMEOUT_S = 300
 
+monkey_patch_torch_compile()
+
 
 class ModelRunner:
     """ModelRunner runs the forward passes of the models."""

python/sglang/srt/patch_torch.py

@@ -14,6 +14,7 @@
 from typing import Callable, Union
 
 import torch
+from packaging import version
 from torch.multiprocessing import reductions
 
 
@@ -69,3 +70,13 @@ def _device_from_maybe_uuid(device_maybe_uuid: Union[int, str]) -> int:
 
 def _modify_tuple(t, index: int, modifier: Callable):
     return *t[:index], modifier(t[index]), *t[index + 1 :]
+
+
+def monkey_patch_torch_compile():
+    if version.parse(torch.__version__) < version.parse("2.8.0"):
+        # These things are cacheable by torch.compile. torch.compile just doesn't know it.
+        # This was fixed in PyTorch 2.8, but until then, we monkey patch.
+        import torch._higher_order_ops.auto_functionalize as af
+
+        af.auto_functionalized_v2._cacheable = True
+        af.auto_functionalized._cacheable = True