[Docs] userful hooks

[chhluo]

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Motivation

Add a tutorial to introduce useful hooks provided by MMDetection and MMCV.

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 repos?
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

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 #7810 (cc1a487) into dev (6dbca90) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #7810      +/-   ##
==========================================
- Coverage   64.87%   64.87%   -0.01%     
==========================================
  Files         352      352              
  Lines       28504    28504              
  Branches     4822     4822              
==========================================
- Hits        18493    18491       -2     
+ Misses       9030     9029       -1     
- Partials      981      984       +3     

Flag	Coverage Δ
unittests	64.85% <ø> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/core/hook/memory_profiler_hook.py	95.65% <ø> (ø)
mmdet/core/bbox/samplers/random_sampler.py	75.00% <0.00%> (-5.56%)	⬇️
mmdet/utils/setup_env.py	88.00% <0.00%> (-4.00%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	87.35% <0.00%> (-2.30%)	⬇️
mmdet/datasets/builder.py	52.08% <0.00%> (-1.05%)	⬇️
mmdet/core/bbox/assigners/sim_ota_assigner.py	83.63% <0.00%> (+3.63%)	⬆️

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[jbwang1997]

LGTM