fix(tests): import deepmd.pt before load_custom_ops in gen_model_devi

All other gen scripts import from deepmd.pt.utils.serialization before
calling load_custom_ops(), which loads the installed libdeepmd_op_pt.so
first. The guard in load_custom_ops() then skips the build-dir copy.

gen_model_devi.py was missing this import, so load_custom_ops() loaded
the build-dir .so first, then the later deepmd.infer import loaded the
installed .so — triggering a duplicate op registration crash.

Author: Han Wang

source/tests/infer/gen_model_devi.py

@@ -66,6 +66,9 @@ def main():
     }
 
     # ---- 2. Build two models with different seeds ----
+    from deepmd.pt.utils.serialization import (  # noqa: F401
+        deserialize_to_file,
+    )
     from deepmd.pt_expt.utils.serialization import (
         deserialize_to_file as pt_expt_deserialize_to_file,
     )