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

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

vllm/model_executor/layers/linear.py

@@ -304,6 +304,7 @@ class LinearBase(CustomOp):
                 param.tp_size = self.tp_size
 
 
+# --8<-- [start:replicated_linear]
 @CustomOp.register("replicated_linear")
 class ReplicatedLinear(LinearBase):
     """Replicated linear layer.
@@ -321,6 +322,8 @@ class ReplicatedLinear(LinearBase):
         disable_tp: Take no effect for replicated linear layers.
     """
 
+    # --8<-- [end:replicated_linear]
+
     def __init__(
         self,
         input_size: int,
@@ -421,6 +424,7 @@ class ReplicatedLinear(LinearBase):
         return s
 
 
+# --8<-- [start:column_parallel_linear]
 @CustomOp.register("column_parallel_linear")
 class ColumnParallelLinear(LinearBase):
     """Linear layer with column parallelism.
@@ -448,6 +452,8 @@ class ColumnParallelLinear(LinearBase):
         disable_tp: If true, weights matrix won't be sharded through tp rank.
     """
 
+    # --8<-- [end:column_parallel_linear]
+
     def __init__(
         self,
         input_size: int,
@@ -1289,6 +1295,7 @@ class QKVParallelLinear(ColumnParallelLinear):
         param_data.copy_(loaded_weight)
 
 
+# --8<-- [start:row_parallel_linear]
 @CustomOp.register("row_parallel_linear")
 class RowParallelLinear(LinearBase):
     """Linear layer with row parallelism.
@@ -1323,6 +1330,8 @@ class RowParallelLinear(LinearBase):
         disable_tp: If true, weights matrix won't be sharded through tp rank.
     """
 
+    # --8<-- [end:row_parallel_linear]
+
     def __init__(
         self,
         input_size: int,