Export single output only (#7259)

* Update

* Update

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

export.py

@@ -477,6 +477,7 @@ def run(
         if isinstance(m, Detect):
             m.inplace = inplace
             m.onnx_dynamic = dynamic
+            m.export = True
             if hasattr(m, 'forward_export'):
                 m.forward = m.forward_export  # assign custom forward (optional)