Fix serialization issue with W&B (#5693)

The chain input_documents are not displaying properly in W&B, due to
serialization issue:

<img width="1164" alt="Screenshot 2023-06-04 at 11 58 26 AM"
src="https://github.com/hwchase17/langchain/assets/134809928/f31f14f6-0935-4cca-9913-6760cd40eadf">

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/callbacks/tracers/wandb.py

@@ -60,10 +60,20 @@ def _convert_llm_run_to_wb_span(trace_tree: Any, run: Run) -> trace_tree.Span:
     return base_span
 
 
+def _serialize_inputs(run_inputs: dict) -> Union[dict, list]:
+    if "input_documents" in run_inputs:
+        docs = run_inputs["input_documents"]
+        return [doc.json() for doc in docs]
+    else:
+        return run_inputs
+
+
 def _convert_chain_run_to_wb_span(trace_tree: Any, run: Run) -> trace_tree.Span:
     base_span = _convert_run_to_wb_span(trace_tree, run)
 
-    base_span.results = [trace_tree.Result(inputs=run.inputs, outputs=run.outputs)]
+    base_span.results = [
+        trace_tree.Result(inputs=_serialize_inputs(run.inputs), outputs=run.outputs)
+    ]
     base_span.child_spans = [
         _convert_lc_run_to_wb_span(trace_tree, child_run)
         for child_run in run.child_runs
@@ -79,7 +89,9 @@ def _convert_chain_run_to_wb_span(trace_tree: Any, run: Run) -> trace_tree.Span:
 
 def _convert_tool_run_to_wb_span(trace_tree: Any, run: Run) -> trace_tree.Span:
     base_span = _convert_run_to_wb_span(trace_tree, run)
-    base_span.results = [trace_tree.Result(inputs=run.inputs, outputs=run.outputs)]
+    base_span.results = [
+        trace_tree.Result(inputs=_serialize_inputs(run.inputs), outputs=run.outputs)
+    ]
     base_span.child_spans = [
         _convert_lc_run_to_wb_span(trace_tree, child_run)
         for child_run in run.child_runs