Add model.eval() in neural_style_tutorial.py

[Suhas-G]

Fixes #1736

Description

Set model to evaluation mode along with requires_grad_(False) in Neural Style Transfer tutorial.

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 unnessessary issues are included into this pull request.

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