Refactor autoaugment: extract _AutoAugmentBase to deduplicate fill logic

[sunnycho100]

Closes #9446

Motivation

There is a FIXME at line 103 of torchvision/transforms/autoaugment.py asking to eliminate copy-pasted fill standardization code. Four classes (AutoAugment, RandAugment, TrivialAugmentWide, AugMix) all duplicate the same fill-processing block in their forward() methods and independently set interpolation/fill in their init. The v2 version already solved this with _AutoAugmentBase - this brings the same pattern to v1.

Changes

• Added _AutoAugmentBase(torch.nn.Module) with common init for interpolation/fill and a _get_fill() helper
• Updated all 4 classes to inherit from _AutoAugmentBase instead of torch.nn.Module
• Each class delegates interpolation/fill to super().init() and calls self._get_fill(img) in forward()
• Removed the FIXME comment

Not changed

• _augmentation_space() is left per-class (different signatures and contents)
• No public API changes, no behavior changes - pure internal refactor

Backward-compatible: yes, no public API surface is affected.

[pytorch-bot]

🔗 Helpful Links

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❌ 2 New Failures

As of commit 2dcef64 with merge base b358d66 ():

NEW FAILURES - The following jobs have failed:

• Lint / python-source-and-configs / linux-job (gh)
  RuntimeError: Command docker exec -t 99c836611dc5bdacd5e3724199957ff05e764af50f0d881e22db45dc9c817089 /exec failed with exit code 1
• Lint / python-types / linux-job (gh)

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[zy1git]

@sunnycho100 Thanks a lot for opening this PR and for your willingness to contribute to TorchVision!
We won’t be merging the changes in this PR as-is, since we’re focusing on V2 going forward and aren’t planning to make further changes to V1. Also, it looks like this PR includes commits from another PR (#9444).
If you update this PR to only remove the FIXME comment and adjust the PR description accordingly, I’m happy to approve and merge it.