validate source_bundle shebang (in ./train)

[jfri3d]

What is the desired change?

Enforce validation of the ./train executable within the source_bundle to ensure a shebang exists!!!

#!/usr/bin/env XXXXXX

ALSO ensure documentation is adjusted to "remind" users that the Train Pipeline uses ./train as an executable.

How will this change improve kaos?

Improve usability to avoid broken trainings where ./train can't run...

Any thoughts on the implementation approach?

Validate in the backend. Inspiration below.

kaos/backend/kaos_backend/util/validators.py

Line 66 in e638d28

def validate_bundle_structure(cls, directory, req_files):