[Doc] Add developer guide for CustomOp (#30886)

Signed-off-by: shen-shanshan <467638484@qq.com>

vllm/model_executor/models/plamo2.py

@@ -103,8 +103,11 @@ def is_mamba(config: Plamo2Config, i: int) -> bool:
 # Adapted from:
 # vllm.model_executor.layers.mamba.mamba_mixer2.MambaMixer2
 # transformers.models.mamba.modeling_mamba.MambaMixer
-@CustomOp.register(name="plamo2_mamba_mixer")
+# --8<-- [start:plamo2_mamba_mixer]
+@CustomOp.register("plamo2_mamba_mixer")
 class Plamo2MambaMixer(MambaBase, CustomOp):
+    # --8<-- [end:plamo2_mamba_mixer]
+
     def __init__(self, vllm_config: VllmConfig, *, prefix: str = "", **kwargs) -> None:
         super().__init__()
         self.config = vllm_config.model_config.hf_config

vllm/model_executor/models/transformers/moe.py

@@ -37,10 +37,13 @@ if TYPE_CHECKING:
     from vllm.config import VllmConfig
 
 
+# --8<-- [start:transformers_fused_moe]
 @CustomOp.register("transformers_fused_moe")
 class TransformersFusedMoE(FusedMoE):
     """Custom FusedMoE for the Transformers modeling backend."""
 
+    # --8<-- [end:transformers_fused_moe]
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self._topk_ids: torch.Tensor = None