Fix assignment of template field in __init__ in custom_job

[romsharon98]

related: #36484

Fix:

CreateCustomContainerTrainingJobOperator's constructor lacks direct assignments for instance members corresponding to the following template fields (i.e., self.field_name = field_name or 
super.__init__(field_name=field_name, ...) ):
['dataset_id', 'region', 'parent_model', 'impersonation_chain']
CreateCustomPythonPackageTrainingJobOperator's constructor lacks direct assignments for instance members corresponding to the following template fields (i.e., self.field_name = field_name or 
super.__init__(field_name=field_name, ...) ):
['dataset_id', 'region', 'parent_model', 'impersonation_chain']
CreateCustomTrainingJobOperator's constructor lacks direct assignments for instance members corresponding to the following template fields (i.e., self.field_name = field_name or 
super.__init__(field_name=field_name, ...) ):
['dataset_id', 'region', 'parent_model', 'impersonation_chain']

cc: @shahar1

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[potiuk]

Conflicts need resolving.