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

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

vllm/model_executor/layers/layernorm.py

@@ -88,6 +88,7 @@ def dispatch_rocm_rmsnorm_func(
     return rms_norm
 
 
+# --8<-- [start:rms_norm]
 @CustomOp.register("rms_norm")
 class RMSNorm(CustomOp):
     """Root mean square normalization.
@@ -96,6 +97,8 @@ class RMSNorm(CustomOp):
     Refer to https://arxiv.org/abs/1910.07467
     """
 
+    # --8<-- [end:rms_norm]
+
     def __init__(
         self,
         hidden_size: int,
@@ -253,6 +256,7 @@ class RMSNorm(CustomOp):
         return s
 
 
+# --8<-- [start:gemma_rms_norm]
 @CustomOp.register("gemma_rms_norm")
 class GemmaRMSNorm(CustomOp):
     """RMS normalization for Gemma.
@@ -262,6 +266,8 @@ class GemmaRMSNorm(CustomOp):
         2. (x * w).to(orig_dtype) instead of x.to(orig_dtype) * w.
     """
 
+    # --8<-- [end:gemma_rms_norm]
+
     def __init__(
         self,
         hidden_size: int,
@@ -321,6 +327,7 @@ class GemmaRMSNorm(CustomOp):
         return self.forward_native(x, residual)
 
 
+# --8<-- [start:rms_norm_gated]
 @CustomOp.register("rms_norm_gated")
 class RMSNormGated(CustomOp):
     """RMS Normalization with optional gating.
@@ -331,6 +338,8 @@ class RMSNormGated(CustomOp):
     - Optional gating with SiLU activation
     """
 
+    # --8<-- [end:rms_norm_gated]
+
     def __init__(
         self,
         hidden_size: int,