Modernize Neural Networks tutorial patterns

[ymrohit]

Fixes #3877

Description

Modernizes the Neural Networks tutorial by updating the old-style super() call, replacing the commented .data parameter update with a torch.no_grad() update, and clarifying why net.zero_grad() is still used before the optimizer section introduces optimizer.zero_grad().

Testing

• git diff --check origin/main..HEAD
• python -m py_compile beginner_source/blitz/neural_networks_tutorial.py
• python beginner_source/blitz/neural_networks_tutorial.py

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3881

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