fix(pt): fix model_def_script

[njzjz]

When executing

cd examples/water/se_e2_a
dp --pt train input.json
dp --pt freeze
dp convert-backend fromzen_model.pth frozen_model.pb
dp convert-backend fromzen_model.pb frozen_model.pth

An error throws in the last step:

TypeError: deepmd.tf.descriptor.descriptor.Descriptor() got multiple values for keyword argument 'ntypes'

The reason is that model_def_script is modified and further saved

deepmd-kit/deepmd/pt/model/model/__init__.py

Line 145 in 3b230c2

model_params["descriptor"]["ntypes"] = ntypes

[njzjz]

In the current test, there are only tests for TF-TF, DP-DP, PT-PT, TF-DP, and PT-DP, but no tests for PT-TF, so it's impossible to find this issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.87%. Comparing base (3eb3d51) to head (3b230c2).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3671   +/-   ##
=======================================
  Coverage   81.87%   81.87%           
=======================================
  Files         503      503           
  Lines       46570    46571    +1     
  Branches     2952     2952           
=======================================
+ Hits        38129    38130    +1     
  Misses       7548     7548           
  Partials      893      893           

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