len needs to be implemented in all memory classes (#236)

* len needs to be implemented in all memory classes so that the pretty print of memory shows statistics

* stub

memgpt/memory.py

@@ -172,6 +172,11 @@ async def a_summarize_messages(
 
 
 class ArchivalMemory(ABC):
+    @abstractmethod
+    def __len__(self):
+        """Define the length of the object. Must be implemented by subclasses."""
+        pass
+
     @abstractmethod
     def insert(self, memory_string):
         """Insert new archival memory
@@ -439,6 +444,11 @@ class DummyArchivalMemoryWithFaiss(DummyArchivalMemory):
 
 
 class RecallMemory(ABC):
+    @abstractmethod
+    def __len__(self):
+        """Define the length of the object. Must be implemented by subclasses."""
+        pass
+
     @abstractmethod
     def text_search(self, query_string, count=None, start=None):
         pass
@@ -680,6 +690,10 @@ class LocalArchivalMemory(ArchivalMemory):
         # TODO: have some mechanism for cleanup otherwise will lead to OOM
         self.cache = {}
 
+    def __len__(self):
+        # TODO FIXME
+        return 1
+
     def save(self):
         """Save the index to disk"""
         if self.agent_config.data_source:  # update original archival index