Added functionality to have multiple target for regression task

Description

Added a functionality to use an alternate target based on condition set.

For example, in jet pT regression task, if we would like to replace the original truth pT target pt_a to pt_b when the jets have a label that is not equal to 4. This can be done by having the following config.

data:
  multi_target:
      - input_name: jets
        sel_label: label
        opp: '!='
        value: 4
        old: pT_a
        new: pT_b

We can also use this functionality to build custom targets based on the condition.

For example, in GN3 pT regression, if we would like to add a pT regression task specifically for \tau jets. We can set up the task config in the following way

- class_path: salt.models.RegressionTask
  init_args:
    name: regression_with_custom_labels
    input_name: jets
    targets: pt_label_handle

Then with the following multi_target config block

data:
  multi_target:
    - input_name: jets
      sel_label: HadronGhostTruthLabelID
      opp: '=='
      value: 15
      old: pt_label_handle
      new: pt_visFromTruthTaus

The label pt_label_handle would take the value of pt_visFromTruthTaus for \tau jets and have the remaining target set to be nans.

Todo:

• Write a test
• Documentation

Review checklist:

• CI Passing
• Comments addressed
• Source branch is up to date with target