[PT FE] Support axis=None for aten::mean operation

[mvafin]

Details:

• Support axis=None for aten::mean operation

Tickets:

• CVS-176842

[Copilot]

Pull Request Overview

This PR adds support for axis=None in the aten::mean operation for PyTorch frontend, allowing the mean to be computed over all dimensions when no axis is specified.

Key changes:

• Modified the C++ mean operation handler to default to all axes when axis parameter is None
• Updated test cases to validate the new axis=None behavior and fixed test function naming
• Improved code readability with better formatting and removal of redundant logic

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/frontends/pytorch/src/op/mean.cpp	Added default initialization of axes to cover all dimensions and conditional handling for None axis input
tests/layer_tests/pytorch_tests/test_mean.py	Fixed test function name from test_sum to test_mean, added axis=None test case, improved code formatting, and fixed parameter handling logic

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