core[minor], community[patch], langchain[patch]: move BaseChatLoader to core (#19607)

Thank you for contributing to LangChain!

- [ ] **PR title**: "core: move BaseChatLoader and BaseToolkit from
community"


- [ ] **PR message**: move BaseChatLoader and BaseToolkit

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/core/langchain_core/chat_loaders.py

@@ -0,0 +1,16 @@
+from abc import ABC, abstractmethod
+from typing import Iterator, List
+
+from langchain_core.chat_sessions import ChatSession
+
+
+class BaseChatLoader(ABC):
+    """Base class for chat loaders."""
+
+    @abstractmethod
+    def lazy_load(self) -> Iterator[ChatSession]:
+        """Lazy load the chat sessions."""
+
+    def load(self) -> List[ChatSession]:
+        """Eagerly load the chat sessions into memory."""
+        return list(self.lazy_load())