Revert "improve the performance of base.py" (#11143)

Reverts langchain-ai/langchain#8610

this is actually an oversight - this merges all dfs into one df. we DO
NOT want to do this - the idea is we work and manipulate multiple dfs

libs/langchain/langchain/agents/agent_toolkits/csv/base.py

@@ -24,13 +24,11 @@ def create_csv_agent(
     if isinstance(path, (str, IOBase)):
         df = pd.read_csv(path, **_kwargs)
     elif isinstance(path, list):
-        if not all(isinstance(item, (str, IOBase)) for item in path):
-            raise ValueError(
-                f"Expected all elements in the list to be strings, got {type(path)}."
-            )
-        dfs = [pd.read_csv(item, **_kwargs) for item in path]
-        df = pd.concat(dfs, ignore_index=True)
+        df = []
+        for item in path:
+            if not isinstance(item, (str, IOBase)):
+                raise ValueError(f"Expected str or file-like object, got {type(path)}")
+            df.append(pd.read_csv(item, **_kwargs))
     else:
-        raise ValueError(f"Expected str or list, got {type(path)}")
-
+        raise ValueError(f"Expected str, list, or file-like object, got {type(path)}")
     return create_pandas_dataframe_agent(llm, df, **kwargs)