[BUG] Cannot convert dipole model from TF to PT

[njzjz]

Bug summary

An error throws when using dp convert-backend to convert a frozen dipole model from TF .pb format to PT .pth format.

DeePMD-kit Version

3eb3d51 (edit mode)

Backend and its version

TF 2.15.0, PT 2.1.2.post300

How did you download the software?

Built from source

Input Files, Running Commands, Error Log, etc.

Traceback (most recent call last):
  File "/home/jz748/anaconda3/bin/dp", line 8, in <module>
    sys.exit(main())
  File "/home/jz748/codes/deepmd-kit/deepmd/main.py", line 807, in main
    deepmd_main(args)
  File "/home/jz748/codes/deepmd-kit/deepmd/entrypoints/main.py", line 83, in main
    convert_backend(**dict_args)
  File "/home/jz748/codes/deepmd-kit/deepmd/entrypoints/convert_backend.py", line 26, in convert_backend
    data = inp_hook(INPUT)
  File "/home/jz748/codes/deepmd-kit/deepmd/tf/utils/serialization.py", line 47, in serialize_from_file
    model_dict = model.serialize()
  File "/home/jz748/codes/deepmd-kit/deepmd/tf/model/model.py", line 825, in serialize
    dict_fit = self.fitting.serialize(suffix=suffix)
  File "/home/jz748/codes/deepmd-kit/deepmd/tf/fit/dipole.py", line 375, in serialize
    "nets": self.serialize_network(
  File "/home/jz748/codes/deepmd-kit/deepmd/tf/fit/fitting.py", line 206, in serialize_network
    return fittings.serialize()
  File "/home/jz748/codes/deepmd-kit/deepmd/dpmodel/utils/network.py", line 769, in serialize
    "networks": [nn.serialize() for nn in self._networks],
  File "/home/jz748/codes/deepmd-kit/deepmd/dpmodel/utils/network.py", line 769, in <listcomp>
    "networks": [nn.serialize() for nn in self._networks],
AttributeError: 'NoneType' object has no attribute 'serialize'

Steps to Reproduce

cd examples/water_tensor/dipole
dp --tf train dipole_input.json
dp --tf freeze
dp convert-backend frozen_model.pb frozen_model.pth 

Further Information, Files, and Links

No response

[njzjz]

I think I got the reason. Only types in sel_type have networks.