fix block.export (#17970) (#19075)

This PR cherry-picks commit 5122d32 into the v1.x branch. This is to enable the export of models where dangling layers are optimized out during symbol export. For more information, see here and here.

python/mxnet/gluon/block.py

@@ -1238,8 +1238,11 @@ def export(self, path, epoch=0, remove_amp_cast=True):
                 if name in arg_names:
                     arg_dict['arg:{}'.format(name)] = param._reduce()
                 else:
-                    assert name in aux_names
-                    arg_dict['aux:{}'.format(name)] = param._reduce()
+                    if name not in aux_names:
+                        warnings.warn('Parameter "{name}" is not found in the graph. '
+                                      .format(name=name), stacklevel=3)
+                    else:
+                        arg_dict['aux:%s'%name] = param._reduce()
         save_fn = _mx_npx.save if is_np_array() else ndarray.save
         save_fn('%s-%04d.params'%(path, epoch), arg_dict)