Add channel_axis to random_flip_left_right

[v1shay]

Summary

• add the documented channel_axis argument to random_flip_left_right
• forward the axis to flip_left_right
• add a regression test for channel-first (CHW) images

Motivation

The deterministic flip_left_right API already supports channel-first images through channel_axis, but its random wrapper did not expose that argument. This made the random API inconsistent and prevented callers from using HWC/CHW layouts interchangeably.

Validation

• python3.12 -m py_compile dm_pix/_src/augment.py dm_pix/_src/augment_test.py
• git diff --check

The full test suite was not run because the local TensorFlow test dependency installation did not complete in this environment.

This PR resolves #111.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.