(manually) update zero gradients after updating the weights

[bintang-aswam]

Fixes #ISSUE_NUMBER : PyTorch Docathon H1 2025 #153952
file location: https://github.com/pytorch/tutorials/blob/main/beginner_source/examples_autograd/polynomial_custom_function.py

Description

Previous version:
a.grad, b.grad, c.grad, d.grad = None, None, None, None

instead it's appropriate to just set 0. instead of "None" as follows:
a.grad, b.grad, c.grad, d.grad = 0. , 0. , 0. , 0.

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/3396

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❌ 1 New Failure

As of commit 268af8f with merge base ab2aafd ():

NEW FAILURE - The following job has failed:

• Docathon Labels Sync / check-labels (gh)
  Process completed with exit code 1.

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