[Fix] fix ssim only calculate on the first channel bug

[Z-Fran]

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Motivation

def img_transform(img,
                  crop_border=0,
                  input_order='HWC',
                  convert_to=None,
                  channel_order='rgb'):
    ....

    if crop_border != 0:
        img = img[crop_border:-crop_border, crop_border:-crop_border, None]

    return img

If crop_border is not 0, the image is wrongly expanded at the second dimension.
All SSIM evaluation with convert_to=None and crop_border!=0 setting will trigger this bug.

Modification

fix img_transform function

Who can help? @ them here!

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.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 88.03% // Head: 88.03% // No change to project coverage 👍

Coverage data is based on head (2f1fbbb) compared to base (4ee8e91).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1515   +/-   ##
========================================
  Coverage    88.03%   88.03%           
========================================
  Files          383      383           
  Lines        24176    24176           
  Branches      3793     3793           
========================================
  Hits         21283    21283           
  Misses        2077     2077           
  Partials       816      816           

Flag	Coverage Δ
unittests	88.03% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmedit/evaluation/metrics/metrics_utils.py	90.00% <100.00%> (ø)

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