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## Why are these changes needed?

<!-- Please give a short summary of the change and the problem this
solves. -->
Shows an example of how to use the `Memory` interface to implement a
just-in-time vector memory based on chromadb.

```python
import os
from pathlib import Path

from autogen_agentchat.agents import AssistantAgent
from autogen_agentchat.ui import Console
from autogen_core.memory import MemoryContent, MemoryMimeType
from autogen_ext.memory.chromadb import ChromaDBVectorMemory, PersistentChromaDBVectorMemoryConfig
from autogen_ext.models.openai import OpenAIChatCompletionClient

# Initialize ChromaDB memory with custom config
chroma_user_memory = ChromaDBVectorMemory(
    config=PersistentChromaDBVectorMemoryConfig(
        collection_name="preferences",
        persistence_path=os.path.join(str(Path.home()), ".chromadb_autogen"),
        k=2,  # Return top  k results
        score_threshold=0.4,  # Minimum similarity score
    )
)
# a HttpChromaDBVectorMemoryConfig is also supported for connecting to a remote ChromaDB server

# Add user preferences to memory
await chroma_user_memory.add(
    MemoryContent(
        content="The weather should be in metric units",
        mime_type=MemoryMimeType.TEXT,
        metadata={"category": "preferences", "type": "units"},
    )
)

await chroma_user_memory.add(
    MemoryContent(
        content="Meal recipe must be vegan",
        mime_type=MemoryMimeType.TEXT,
        metadata={"category": "preferences", "type": "dietary"},
    )
)


# Create assistant agent with ChromaDB memory
assistant_agent = AssistantAgent(
    name="assistant_agent",
    model_client=OpenAIChatCompletionClient(
        model="gpt-4o",
    ),
    tools=[get_weather],
    memory=[user_memory],
)

stream = assistant_agent.run_stream(task="What is the weather in New York?")
await Console(stream)

await user_memory.close()
```

```txt
 ---------- user ----------
What is the weather in New York?
---------- assistant_agent ----------
[MemoryContent(content='The weather should be in metric units', mime_type='MemoryMimeType.TEXT', metadata={'category': 'preferences', 'mime_type': 'MemoryMimeType.TEXT', 'type': 'units', 'score': 0.4342913043162201, 'id': '8a8d683c-5866-41e1-ac17-08c4fda6da86'}), MemoryContent(content='The weather should be in metric units', mime_type='MemoryMimeType.TEXT', metadata={'category': 'preferences', 'mime_type': 'MemoryMimeType.TEXT', 'type': 'units', 'score': 0.4342913043162201, 'id': 'f27af42c-cb63-46f0-b26b-ffcc09955ca1'})]
---------- assistant_agent ----------
[FunctionCall(id='call_a8U3YEj2dxA065vyzdfXDtNf', arguments='{"city":"New York","units":"metric"}', name='get_weather')]
---------- assistant_agent ----------
[FunctionExecutionResult(content='The weather in New York is 23 °C and Sunny.', call_id='call_a8U3YEj2dxA065vyzdfXDtNf', is_error=False)]
---------- assistant_agent ----------
The weather in New York is 23 °C and Sunny.
```

Note that MemoryContent object in the MemoryQuery events have useful
metadata like the score and id retrieved memories.

## Related issue number

<!-- For example: "Closes #1234" -->


## Checks

- [ ] I've included any doc changes needed for
https://microsoft.github.io/autogen/. See
https://microsoft.github.io/autogen/docs/Contribute#documentation to
build and test documentation locally.
- [ ] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [ ] I've made sure all auto checks have passed.