Add BaseCallbackHandler and CallbackManager (#478)

Co-authored-by: Ankush Gola <9536492+agola11@users.noreply.github.com>

tests/unit_tests/llms/test_callbacks.py

@@ -0,0 +1,30 @@
+"""Test LLM callbacks."""
+from langchain.callbacks.base import CallbackManager
+from tests.unit_tests.callbacks.fake_callback_handler import FakeCallbackHandler
+from tests.unit_tests.llms.fake_llm import FakeLLM
+
+
+def test_llm_with_callbacks() -> None:
+    """Test LLM callbacks."""
+    handler = FakeCallbackHandler()
+    llm = FakeLLM(callback_manager=CallbackManager(handlers=[handler]), verbose=True)
+    output = llm("foo")
+    assert output == "foo"
+    assert handler.starts == 1
+    assert handler.ends == 1
+    assert handler.errors == 0
+
+
+def test_llm_with_callbacks_not_verbose() -> None:
+    """Test LLM callbacks but not verbose."""
+    import langchain
+
+    langchain.verbose = False
+
+    handler = FakeCallbackHandler()
+    llm = FakeLLM(callback_manager=CallbackManager(handlers=[handler]))
+    output = llm("foo")
+    assert output == "foo"
+    assert handler.starts == 0
+    assert handler.ends == 0
+    assert handler.errors == 0