Add read only shared memory (#1491)

Provide shared memory capability for the Agent.
Inspired by #1293 .

## Problem

If both Agent and Tools (i.e., LLMChain) use the same memory, both of
them will save the context. It can be annoying in some cases.


## Solution

Create a memory wrapper that ignores the save and clear, thereby
preventing updates from Agent or Tools.

tests/unit_tests/chains/test_conversation.py

@@ -34,7 +34,7 @@ def test_conversation_chain_works() -> None:
 
 
 def test_conversation_chain_errors_bad_prompt() -> None:
-    """Test that conversation chain works in basic setting."""
+    """Test that conversation chain raise error with bad prompt."""
     llm = FakeLLM()
     prompt = PromptTemplate(input_variables=[], template="nothing here")
     with pytest.raises(ValueError):
@@ -42,7 +42,7 @@ def test_conversation_chain_errors_bad_prompt() -> None:
 
 
 def test_conversation_chain_errors_bad_variable() -> None:
-    """Test that conversation chain works in basic setting."""
+    """Test that conversation chain raise error with bad variable."""
     llm = FakeLLM()
     prompt = PromptTemplate(input_variables=["foo"], template="{foo}")
     memory = ConversationBufferMemory(memory_key="foo")

tests/unit_tests/chains/test_memory.py

@@ -1,4 +1,13 @@
-from langchain.memory.simple import SimpleMemory
+import pytest
+
+from langchain.chains.conversation.memory import (
+    ConversationBufferMemory,
+    ConversationBufferWindowMemory,
+    ConversationSummaryMemory,
+)
+from langchain.memory import ReadOnlySharedMemory, SimpleMemory
+from langchain.schema import BaseMemory
+from tests.unit_tests.llms.fake_llm import FakeLLM
 
 
 def test_simple_memory() -> None:
@@ -9,3 +18,20 @@ def test_simple_memory() -> None:
 
     assert output == {"baz": "foo"}
     assert ["baz"] == memory.memory_variables
+
+
+@pytest.mark.parametrize(
+    "memory",
+    [
+        ConversationBufferMemory(memory_key="baz"),
+        ConversationSummaryMemory(llm=FakeLLM(), memory_key="baz"),
+        ConversationBufferWindowMemory(memory_key="baz"),
+    ],
+)
+def test_readonly_memory(memory: BaseMemory) -> None:
+    read_only_memory = ReadOnlySharedMemory(memory=memory)
+    memory.save_context({"input": "bar"}, {"output": "foo"})
+
+    assert read_only_memory.load_memory_variables({}) == memory.load_memory_variables(
+        {}
+    )