[Term Entry] PyTorch Tensors .mean()

[codecademy-docs]

Reason/inspiration (optional)

Add a new term entry about the .mean() method for Tensors. Place your file under the file path docs/content/pytorch/concepts/tensors/mean/mean.md

It should include the following sections:

• A "Syntax" section with a ```pseudo code block
• An "Example" section with a ```py code block
• A "Codebyte Example" section with a ```codebyte/python code block

Please refer to the concept entry templates, content standards, and markdown style guide when working on the PR for this issue.

Entry Type (select all that apply)

New Term for Existing Concept Entry

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