Convert computational graph diagrams to Mermaid

[saurabhkthakur]

Fixes #3580

Description

• corrected requires_grad value for z in computational graph
• recreated both computational graph diagrams using Mermaid

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/3873

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure

As of commit 447ffa2 with merge base 08435cc ():

NEW FAILURE - The following job has failed:

• Build tutorials (nightly/test) / build / pytorch_tutorial_build_worker (11, 15, linux.g5.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.

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