Update yolov5 yaml build to handle case where directory with yaml is provided

[dsikka]

For this ticket: https://app.asana.com/0/1204322589046915/1205045628929361/f

Previously we handled 2 cases:

• User provides path to a yaml
• User provides a directory of custom data + classes and we build the yaml for them

This adds the third case where the user provides a directory and we first check if a yaml already exists in the folder. If there is a yaml, we use that as the data.

Tested locally using a training-aware run