Fix calculation of current item number in optimization_tutorial.py

[anttikup]

Current item calculation gave the wrong number when dataset doesn't split to batches equally.

Fixes #2742

Description

Change (batch + 1) * len(X) to batch * batch_size + len(X)

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.

cc @subramen @albanD

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/2743

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✅ You can merge normally! (4 Unrelated Failures)

As of commit 0331944 with merge base 49c1494 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Build tutorials / pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.
• Build tutorials / pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.
• Build tutorials / pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.
• Build tutorials / pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.

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[albanD]

Thanks