[Fix] Update mmcv version limitation and Readme.

[LeoXing1996]

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Motivation

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Modification

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BC-breaking (Optional)

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Use cases (Optional)

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Checklist

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• 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.

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[codecov]

Codecov Report

Base: 74.75% // Head: 74.75% // No change to project coverage 👍

Coverage data is based on head (3b538af) compared to base (893fc6b).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 3b538af differs from pull request most recent head 3b2399b. Consider uploading reports for the commit 3b2399b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   74.75%   74.75%           
=======================================
  Files         145      145           
  Lines       10913    10913           
  Branches     2145     2145           
=======================================
  Hits         8158     8158           
  Misses       2220     2220           
  Partials      535      535           

Flag	Coverage Δ
unittests	74.75% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmgen/version.py	58.33% <100.00%> (ø)

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