Fix handling of training step dependencies to allow successful pipeline creation

[bobbywlindsey]

Issue #, if available:

Description of changes: Fix handling of dependencies in get_training_code_hash workflow utility so that training steps without a requirements param specified will still create a pipeline.

Originally, dependencies param of function get_training_code_hash assumed a list of strings. But for the training step, this is actually referring to requirements which can be a string, pipeline variable, or None - not a list. When this function is called, it tries to return hash_files_or_dirs([source_dir] + dependencies) but throws a list concatenation error because dependencies will always be either a string, pipeline variable, or None.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[mollyheamazon]

Please add unit test to your fix