[Fix] Avoid installing both opencv and opencv-headless.

[mzr1996]

Motivation

Our optional dependency albumentations may install both opencv and opencv-headless. Refers to the docs.

Modification

This PR updates the installation of albumentations to prevent using a wheel distribution of it with opencv-headless dependency. Refers to open-mmlab/mmpose#833

BC-breaking (Optional)

No

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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

[codecov]

Codecov Report

Merging #397 (d7e6197) into master (a747458) will increase coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head d7e6197 differs from pull request most recent head ba7717c. Consider uploading reports for the commit ba7717c to get more accurate results

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   76.51%   76.54%   +0.02%     
==========================================
  Files          97       97              
  Lines        5224     5222       -2     
  Branches      856      855       -1     
==========================================
  Hits         3997     3997              
+ Misses       1108     1106       -2     
  Partials      119      119              

Flag	Coverage Δ
unittests	76.54% <0.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/datasets/imagenet.py	25.53% <0.00%> (+1.04%)	⬆️

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[Ezra-Yu]

LGTM