Move argument parsing in trainer.py from the main method.

[varisd]

Currently, the command-line argument parsing is performed inside the module's main method. This makes using trainer.main directly in other Python applications difficult.

I suggest moving the argparse part of the code to a separate method (e.g. parse_args) and adding a argparse.Namespace argument to the main method. This way, users can call main method directly from their Python code given that they prepare the input Namespace argument beforehand.