[PD] Add doc and simplify sender.send (#6019)

python/sglang/srt/disaggregation/prefill.py

@@ -384,11 +384,10 @@ class SchedulerDisaggregationPrefillMixin:
             if end_idx is not None
             else min(len(req.fill_ids), len(req.origin_input_ids))
         )
+
         last_chunk = token_id is not None
 
-        if (not last_chunk) and (
-            end_idx % page_size != 0
-        ):  # todo: remove the second condition
+        if not last_chunk:
             # if not the last chunk and the last page is partial, delay the last partial page to the next send
             end_idx = end_idx - end_idx % page_size
 
@@ -405,16 +404,10 @@ class SchedulerDisaggregationPrefillMixin:
                 req.metadata_buffer_index, token_id
             )
         page_indices = kv_to_page_indices(kv_indices, page_size)
-
-        page_start_idx = start_idx // page_size
-        page_end_idx = page_start_idx + len(page_indices)
-
         if len(page_indices) == 0:
             logger.info(
                 f"Skip sending kv chunk for request {req.rid=} {req.bootstrap_room=} because page_indices is empty"
             )
             return
 
-        req.disagg_kv_sender.send(
-            page_indices, slice(page_start_idx, page_end_idx), last_chunk
-        )
+        req.disagg_kv_sender.send(page_indices)