[Enhancement ] Register compose in TRANSFORMS

[HAOCHENYE]

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Motivation

Support using Compose in config file, this requirement is common for TTA:

tta_pipeline = [
    dict(type='LoadImageFromFile', file_client_args=dict(backend='disk')),
    dict(
        type='TestTimeAug',
        transforms=[
            [
                dict(
                    type='Compose',
                    transforms=[
                        dict(type='YOLOv5KeepRatioResize', scale=s),
                        dict(type='LetterResize', scale=s, allow_scale_up=False, pad_val=dict(img=114))
                        for s in img_scales
                    ],
                )
            ]

            [
                dict(type='mmdet.RandomFlip', prob=1.),
                dict(type='mmdet.RandomFlip', prob=0.)
            ],
            [
                dict(type='mmdet.LoadAnnotations', with_bbox=True),
            ],
            
            [
                dict(
                    type='mmdet.PackDetInputs',
                    meta_keys=(
                        'img_id', 'img_path', 'ori_shape', 'img_shape',
                        'scale_factor', 'pad_param', 'flip', 'flip_direction'
                    )
                )
            ]
        ])
]

YOLOv5KeepRatioResize and LetterResize is a combination and should be wrapper by Compose

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 66.39% // Head: 66.39% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9a7f817) compared to base (834f94d).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 9a7f817 differs from pull request most recent head a7ad6d8. Consider uploading reports for the commit a7ad6d8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #2600   +/-   ##
=======================================
  Coverage   66.39%   66.39%           
=======================================
  Files         121      121           
  Lines        7971     7972    +1     
  Branches     1100     1101    +1     
=======================================
+ Hits         5292     5293    +1     
  Misses       2495     2495           
  Partials      184      184           

Flag	Coverage Δ
unittests	66.39% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/transforms/wrappers.py	94.90% <100.00%> (+0.02%)	⬆️

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