feat: prune summary buffer (#4004)

If the library user has to decrease the `max_token_limit`, he would
probably want to prune the summary buffer even though he haven't added
any new messages.

Personally, I need it because I want to serialise memory buffer object
and save to database, and when I load it, I may have re-configured my
code to have a shorter memory to save on tokens.

langchain/memory/summary_buffer.py

@@ -57,7 +57,10 @@ class ConversationSummaryBufferMemory(BaseChatMemory, SummarizerMixin):
     def save_context(self, inputs: Dict[str, Any], outputs: Dict[str, str]) -> None:
         """Save context from this conversation to buffer."""
         super().save_context(inputs, outputs)
-        # Prune buffer if it exceeds max token limit
+        self.prune()
+
+    def prune(self) -> None:
+        """Prune buffer if it exceeds max token limit"""
         buffer = self.chat_memory.messages
         curr_buffer_length = self.llm.get_num_tokens_from_messages(buffer)
         if curr_buffer_length > self.max_token_limit: