feat: pre_trans for variable

[jiangyi15]

Add transform before read variables. We can set two variable to the same, and add transform for one variable to make the constrains of linear transform. For example,

constrains:
    var_equal: [[var_name, var_name2]]
    pre_trans:
         var_name2:
              model: linear
              k: 2.0
              b: 0.0

then var_name2 will be 2 * var_name.

[codecov]

Codecov Report

Patch coverage: 92.66% and project coverage change: +0.16% 🎉

Comparison is base (20ac860) 76.21% compared to head (cf340bb) 76.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #112      +/-   ##
==========================================
+ Coverage   76.21%   76.37%   +0.16%     
==========================================
  Files         108      109       +1     
  Lines       15894    15995     +101     
  Branches     2981     2999      +18     
==========================================
+ Hits        12113    12216     +103     
+ Misses       3084     3079       -5     
- Partials      697      700       +3     

Flag	Coverage Δ
unittests	76.37% <92.66%> (+0.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
tf_pwa/config_loader/config_loader.py	69.16% <88.57%> (+0.78%)	⬆️
tf_pwa/transform.py	91.42% <91.42%> (ø)
tf_pwa/variable.py	60.75% <94.73%> (+1.33%)	⬆️
tf_pwa/tests/test_variable.py	93.89% <100.00%> (+1.10%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Yinrui-Liu]

I agree it's worth adding pre-trans function. For var_equal, you can use VarsManager.set_same(). I think it might be better to show it in configuration file as

constrains:
    pre_trans:
         [var_name2, var_name]:
              model: linear
              k: 2.0
              b: 0.0

and var_equal will be first done implicitly inside pre_trans.

[jiangyi15]

     [var_name2, var_name]:

the list is unhashable, it it unable to use it in the key.
It would be better to used in another special config for connect two variables instead of pre_trans. That support more complex constrains, such as constrains for 3 variables.

[jiangyi15]

I have change it to

constrains:
    from_trans:
         var_name2:
              x:  var_name
              model: linear
              k: 2.0
              b: 0.0