Remove redundant epoch_number variable in training tutorial (issue 3714)

[sheralskumar]

Fixes #3714

Description

Removed redundant epoch_number variable and used the loop variable epoch directly for logging and checkpointing. This simplifies the code, improving clarity for beginners.

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

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NEW FAILURE - The following job has failed:

• Build tutorials (nightly/test) / build / pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu) (gh)
  Process completed with exit code 2.

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[sekyondaMeta]

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PR Review: #3787

Summary

This PR removes the redundant epoch_number variable from beginner_source/introyt/trainingyt.py and replaces all its usages with the epoch loop variable from for epoch in range(EPOCHS). The change is functionally equivalent and simplifies the code.

Content Quality

No concerns. This is a minor cleanup that improves clarity for beginners by eliminating a redundant variable that shadows the loop counter. Previously, epoch_number was initialized to 0 before the loop and incremented at the end of each iteration — producing the exact same sequence as range(EPOCHS). Removing it makes the code easier to follow.

Code Correctness

No concerns. I verified all four substitution sites:

Location	Before	After	Equivalent?
trainingyt.py:286 — print statement	epoch_number + 1	epoch + 1	Yes — both produce 1..5
trainingyt.py:290 — train_one_epoch() call	epoch_number	epoch	Yes — both produce 0..4, used as epoch_index in the function
trainingyt.py:313 — add_scalars() step	epoch_number + 1	epoch + 1	Yes — both produce 1..5
trainingyt.py:319 — model path	epoch_number	epoch	Yes — both produce 0..4

The train_one_epoch function (trainingyt.py:226) receives the value as epoch_index and uses it to compute tb_x = epoch_index * len(training_loader) + i + 1 at line 255. The behavior is unchanged.

Structure & Formatting

No concerns. This is a modification to an existing file with no structural changes needed (no new tutorials, no index/toctree changes).

Build Compatibility

No concerns. The change is purely a variable renaming with identical runtime behavior. No new dependencies, data downloads, or build configuration changes.

Recommendation

Approve

Clean, correct simplification that removes unnecessary complexity from a beginner tutorial. The diff is minimal and behavior-preserving.

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