Implement RunnablePassthrough.pick() (#15184)

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libs/core/tests/unit_tests/runnables/test_runnable.py

@@ -2764,6 +2764,41 @@ def test_map_stream() -> None:
         {"question": "What is your name?"}
     )
 
+    chain_pick_one = chain | RunnablePassthrough.pick("llm")
+
+    stream = chain_pick_one.stream({"question": "What is your name?"})
+
+    final_value = None
+    streamed_chunks = []
+    for chunk in stream:
+        streamed_chunks.append(chunk)
+        if final_value is None:
+            final_value = chunk
+        else:
+            final_value += chunk
+
+    assert streamed_chunks[0] == "i"
+    assert len(streamed_chunks) == len(llm_res)
+
+    chain_pick_two = chain | RunnablePassthrough.pick(["llm", "chat"])
+
+    stream = chain_pick_two.stream({"question": "What is your name?"})
+
+    final_value = None
+    streamed_chunks = []
+    for chunk in stream:
+        streamed_chunks.append(chunk)
+        if final_value is None:
+            final_value = chunk
+        else:
+            final_value += chunk
+
+    assert streamed_chunks[0] in [
+        {"llm": "i"},
+        {"chat": AIMessageChunk(content="i")},
+    ]
+    assert len(streamed_chunks) == len(llm_res) + len(chat_res)
+
 
 def test_map_stream_iterator_input() -> None:
     prompt = (