Bump version to v0.9.1

[fangyixiao18]

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Motivation

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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)

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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.
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[codecov]

Codecov Report

Merging #322 (eeb7047) into master (399b5a0) will increase coverage by 0.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
+ Coverage   72.01%   72.46%   +0.45%     
==========================================
  Files         120      120              
  Lines        4702     4703       +1     
  Branches      757      757              
==========================================
+ Hits         3386     3408      +22     
+ Misses       1162     1137      -25     
- Partials      154      158       +4     

Flag	Coverage Δ
unittests	72.46% <100.00%> (+0.45%)	⬆️

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Impacted Files	Coverage Δ
mmselfsup/core/hooks/cosine_annealing_hook.py	59.45% <ø> (ø)
mmselfsup/core/hooks/__init__.py	100.00% <100.00%> (ø)
mmselfsup/models/utils/transformer_blocks.py	91.71% <100.00%> (+3.02%)	⬆️
mmselfsup/version.py	61.53% <100.00%> (ø)

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