`export.py` automatic `forward_export` (#6352)

* `export.py` automatic `forward_export`

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

for more information, see https://pre-commit.ci

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

export.py

         elif isinstance(m, Detect):
             m.inplace = inplace
             m.onnx_dynamic = dynamic
-            # m.forward = m.forward_export  # assign forward (optional)
+            if hasattr(m, 'forward_export'):
+                m.forward = m.forward_export  # assign custom forward (optional)
 
     for _ in range(2):
         y = model(im)  # dry runs

tutorial.ipynb

       "outputs": []
     }
   ]
-}
+}