[Fix] Fix example

[zhouzaida]

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Motivation

Original example didn't work and it is too complicated for new users. In this PR, I write a simpler example directly run by users.

Modification

• Delete original example
• Add simpler example

BC-breaking (Optional)

No BC-breaking

Use cases (Optional)

Run example

python example/train.py

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1074 (4f08d37) into master (6c63621) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1074   +/-   ##
=======================================
  Coverage   67.99%   67.99%           
=======================================
  Files         159      159           
  Lines       10426    10426           
  Branches     1897     1897           
=======================================
  Hits         7089     7089           
  Misses       2968     2968           
  Partials      369      369           

Flag	Coverage Δ
unittests	67.99% <ø> (ø)

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[hellock]

@mzr1996 may take a look.

[zhouzaida]

@ZwwWayne

[mzr1996]

Should we add a test example? Users cannot evaluate training results from the example now.

[ZwwWayne]

Should we add a test example? Users cannot evaluate training results from the example now.

We can add a test example in a new PR.