adding comments on train.py to make explicit for beginners

[markredito]

Hello!
I'm a beginner in terms of building and training models. I thought that it might be useful to add #comments on train.py on what each component of the code does for easier understanding, especially for people starting out. Here's an example:

If useful, I can do a PR.

Thank you!

[0xdevalias]

See also:

• Code comments #28
• Code Clarity Comments & Style Error Fix In Utils.py #43