update img_shape after Expand

[Mactarvish]

Motivation

Updating img_shape after Expand, otherwise if there's a RandomFlip after Expand the result will be incorrect, because RandomFlip maintains flipped gt_bboxes using result['img_shape'].

Modification

updating img_shape after Expand

BC-breaking (Optional)

None

Use cases (Optional)

train_pipeline = [
    dict(type='LoadImageFromFile', to_float32=True),
    dict(type='LoadAnnotations', with_bbox=True),
    dict( 
        type='Resize', 
        img_scale=[(480 // 2 - 20, 768 // 2 - 20), (480 // 2 + 20, 768 // 2 + 20)],
        multiscale_mode='range', 
        keep_ratio=True),  
    # Using RandomFlip after an Expand directly will course incorrect result before this commit, now it is fixed.
    dict( 
        type='Expand',
        mean=(0,),
        prob=0.5,
        ratio_range=(1.2, 1.5)),
    dict(type='RandomFlip', flip_ratio=0.5, direction=['horizontal']),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='Pad', size_divisor=32),
    dict(type='DefaultFormatBundle'),
    dict(type='Collect', keys=['img', 'gt_bboxes', 'gt_labels']),
]

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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[Mactarvish]

why still not merged yet