Improve LR scheduler documentation in transfer learning tutorial

[sammy-1904]

Description

This PR improves the documentation and comments related to the learning rate scheduler (StepLR) in the transfer_learning_tutorial.py script.

Changes made:

• Added clarification about the recommended order: optimizer.step() followed by scheduler.step().
• Explained why StepLR is stepped once per epoch in this tutorial.
• Improved clarity of the backward/optimizer comment inside the training loop.
• Updated comments to align with current PyTorch best practices.

No functional changes were made. This is a documentation only improvement.

Checklist

• [x] The issue being addressed is documentation clarity (no existing issue number)
• [x] Only one change area is addressed in this PR
• [x] No unnecessary changes are included
• [x] PR follows PyTorch Tutorials contribution guidelines