Update introyt model tutorial APIs

[Vasanthadithya-mundrathi]

Fixes #3856

Description

Updates the Building Models tutorial to use the current PyTorch API style:

• specifies dim=1 for the TinyModel softmax layer
• replaces Python 2-style super(...) calls with super().__init__()

Verification

• python3 -m py_compile beginner_source/introyt/modelsyt_tutorial.py
• MPLBACKEND=Agg /tmp/docathon-torch-venv/bin/python beginner_source/introyt/modelsyt_tutorial.py

Checklist

• The issue that is being fixed is referred in the description
• 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

[pytorch-bot]

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[Copilot]

Pull request overview

Updates the Intro YouTube “Building Models with PyTorch” tutorial code to align with current PyTorch/Python API expectations and eliminate common warnings from deprecated usage.

Changes:

• Updated TinyModel to use torch.nn.Softmax(dim=1) to avoid implicit-dimension deprecation warnings.
• Replaced legacy Python 2-style super(Class, self).__init__() with super().__init__() in TinyModel, LeNet, and LSTMTagger.

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[Vasanthadithya-mundrathi]

Closing in favor of #3895.