fix: assign current_time to datetime.now() if current_time is None (#5045)

# Assign `current_time` to `datetime.now()` if it `current_time is None`
in `time_weighted_retriever`

Fixes #4825 

As implemented, `add_documents` in `TimeWeightedVectorStoreRetriever`
assigns `doc.metadata["last_accessed_at"]` and
`doc.metadata["created_at"]` to `datetime.datetime.now()` if
`current_time` is not in `kwargs`.
```python
    def add_documents(self, documents: List[Document], **kwargs: Any) -> List[str]:
        """Add documents to vectorstore."""
        current_time = kwargs.get("current_time", datetime.datetime.now())
        # Avoid mutating input documents
        dup_docs = [deepcopy(d) for d in documents]
        for i, doc in enumerate(dup_docs):
            if "last_accessed_at" not in doc.metadata:
                doc.metadata["last_accessed_at"] = current_time
            if "created_at" not in doc.metadata:
                doc.metadata["created_at"] = current_time
            doc.metadata["buffer_idx"] = len(self.memory_stream) + i
        self.memory_stream.extend(dup_docs)
        return self.vectorstore.add_documents(dup_docs, **kwargs)
``` 
However, from the way `add_documents` is being called from
`GenerativeAgentMemory`, `current_time` is set as a `kwarg`, but it is
given a value of `None`:
```python
    def add_memory(
        self, memory_content: str, now: Optional[datetime] = None
    ) -> List[str]:
        """Add an observation or memory to the agent's memory."""
        importance_score = self._score_memory_importance(memory_content)
        self.aggregate_importance += importance_score
        document = Document(
            page_content=memory_content, metadata={"importance": importance_score}
        )
        result = self.memory_retriever.add_documents([document], current_time=now)
```
The default of `now` was set in #4658 to be None. The proposed fix is
the following:
```python
    def add_documents(self, documents: List[Document], **kwargs: Any) -> List[str]:
        """Add documents to vectorstore."""
        current_time = kwargs.get("current_time", datetime.datetime.now())
        # `current_time` may exist in kwargs, but may still have the value of None.
        if current_time is None:
            current_time = datetime.datetime.now()
```
Alternatively, we could just set the default of `now` to be
`datetime.datetime.now()` everywhere instead. Thoughts @hwchase17? If we
still want to keep the default to be `None`, then this PR should fix the
above issue. If we want to set the default to be
`datetime.datetime.now()` instead, I can update this PR with that
alternative fix. EDIT: seems like from #5018 it looks like we would
prefer to keep the default to be `None`, in which case this PR should
fix the error.

langchain/retrievers/time_weighted_retriever.py

@@ -109,7 +109,9 @@ class TimeWeightedVectorStoreRetriever(BaseRetriever, BaseModel):
 
     def add_documents(self, documents: List[Document], **kwargs: Any) -> List[str]:
         """Add documents to vectorstore."""
-        current_time = kwargs.get("current_time", datetime.datetime.now())
+        current_time = kwargs.get("current_time")
+        if current_time is None:
+            current_time = datetime.datetime.now()
         # Avoid mutating input documents
         dup_docs = [deepcopy(d) for d in documents]
         for i, doc in enumerate(dup_docs):