[BugFix] Make FP8 Linear compatible with torch.compile (vllm-project#13918)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

Author: WoosukKwon

vllm/model_executor/layers/quantization/fp8.py

@@ -369,12 +369,9 @@ def apply(self,
                 size_k=layer.input_size_per_partition,
                 bias=bias)
 
-        # Note: lazy import to avoid triton import error.
-        from vllm.model_executor.layers.quantization.utils.fp8_utils import (
-            apply_w8a8_block_fp8_linear)
         if self.block_quant:
             assert self.quant_config.weight_block_size is not None
-            return apply_w8a8_block_fp8_linear(
+            return torch.ops.vllm.apply_w8a8_block_fp8_linear(
                 input=x,
                 weight=layer.weight,
                 block_size=self.quant_config.weight_block_size,

vllm/model_executor/layers/quantization/utils/fp8_utils.py

@@ -17,6 +17,7 @@
 from vllm.model_executor.layers.quantization.utils.w8a8_utils import (
     CUTLASS_BLOCK_FP8_SUPPORTED, CUTLASS_FP8_SUPPORTED, apply_fp8_linear)
 from vllm.platforms import current_platform
+from vllm.utils import direct_register_custom_op
 
 logger = init_logger(__name__)
 
@@ -81,6 +82,25 @@ def apply_w8a8_block_fp8_linear(
     return output.to(dtype=input.dtype).view(*output_shape)
 
 
+def apply_w8a8_block_fp8_linear_fake(
+    input: torch.Tensor,
+    weight: torch.Tensor,
+    block_size: List[int],
+    weight_scale: torch.Tensor,
+    input_scale: Optional[torch.Tensor] = None,
+) -> torch.Tensor:
+    output_shape = [*input.shape[:-1], weight.shape[0]]
+    return torch.empty(output_shape, dtype=input.dtype, device=input.device)
+
+
+direct_register_custom_op(
+    op_name="apply_w8a8_block_fp8_linear",
+    op_func=apply_w8a8_block_fp8_linear,
+    mutates_args=[],
+    fake_impl=apply_w8a8_block_fp8_linear_fake,
+)
+
+
 # Unify the interface between `apply_w8a8_block_fp8_linear` and
 # `apply_fp8_linear`
 # NOTE(lucas): this is quite messy, we should think through this more formally