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

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

vllm/model_executor/layers/vocab_parallel_embedding.py

@@ -181,6 +181,7 @@ def get_masked_input_and_mask(
     return input_, ~vocab_mask
 
 
+# --8<-- [start:vocab_parallel_embedding]
 @CustomOp.register("vocab_parallel_embedding")
 class VocabParallelEmbedding(CustomOp):
     """Embedding parallelized in the vocabulary dimension.
@@ -221,6 +222,8 @@ class VocabParallelEmbedding(CustomOp):
         prefix: full name of the layer in the state dict
     """  # noqa: E501
 
+    # --8<-- [end:vocab_parallel_embedding]
+
     def __init__(
         self,
         num_embeddings: int,
@@ -492,6 +495,7 @@ class VocabParallelEmbedding(CustomOp):
         return s
 
 
+# --8<-- [start:parallel_lm_head]
 @CustomOp.register("parallel_lm_head")
 class ParallelLMHead(VocabParallelEmbedding):
     """Parallelized LM head.
@@ -509,6 +513,8 @@ class ParallelLMHead(VocabParallelEmbedding):
         padding_size: padding size for the vocabulary.
     """
 
+    # --8<-- [end:parallel_lm_head]
+
     def __init__(
         self,
         num_embeddings: int,