refactor: Pull out reusable code in CustomTrainingJob to use in other…

… training jobs (#49)

* Extracted reusable CustomTrainingJob code into TrainingJob base class

Moved around some functions

Completed refactor to use base class

bug: remove requirement for import_schema_uri when passing in gcs_source (#46)

Ran linters

Removed model from TrainingJob and moved to CustomTrainingJob

Removed DatasetWithSplits

Added doc strings and simplified training_job_base code

Moved TrainingJob class into training_jobs.py

Removed container_uri from base TrainingJob class

Addressed comments

Fixed managed model

Ran linter

Fixed issues with abc, doc string and super call

Refactored to create input data config separately

* Ran linter

Co-authored-by: Ivan Cheung <[email protected]>