[Enhance] Remove unnecessary calls and lazily import to speed up import performance

[zhouzaida]

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Motivation

Speed the performance when calling import mmengine.runner.

Using pyinstrument to test the difference in performance.

• Before this PR

• After this PR

Modification

1. Remove unnecessary calls to speed import performance

   BuildExtension, CppExtension, CUDAExtension = _get_extension()

   The above line heavily slows the import performance and this line is not necessary. Therefore, remove this line.

2. Layily import torchvision, matplotlib, and pkg_resources to speed the import performance.

BC-breaking (Optional)

No

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

Base: 78.66% // Head: 78.74% // Increases project coverage by +0.08% 🎉

Coverage data is based on head (79d3768) compared to base (fe26c65).
Patch coverage: 48.71% of modified lines in pull request are covered.

❗ Current head 79d3768 differs from pull request most recent head 9ef6e17. Consider uploading reports for the commit 9ef6e17 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #837      +/-   ##
==========================================
+ Coverage   78.66%   78.74%   +0.08%     
==========================================
  Files         128      128              
  Lines        9348     9374      +26     
  Branches     1848     1859      +11     
==========================================
+ Hits         7354     7382      +28     
+ Misses       1679     1678       -1     
+ Partials      315      314       -1     

Flag	Coverage Δ
unittests	78.74% <48.71%> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
mmengine/optim/optimizer/zero_optimizer.py	52.17% <0.00%> (-4.97%)	⬇️
mmengine/registry/build_functions.py	80.20% <ø> (ø)
mmengine/registry/default_scope.py	100.00% <ø> (ø)
mmengine/structures/base_data_element.py	93.65% <ø> (ø)
mmengine/utils/dl_utils/parrots_wrapper.py	44.77% <ø> (-3.76%)	⬇️
mmengine/visualization/vis_backend.py	84.49% <ø> (ø)
mmengine/runner/checkpoint.py	51.75% <10.71%> (+7.79%)	⬆️
mmengine/model/weight_init.py	35.63% <33.33%> (+0.12%)	⬆️
mmengine/visualization/utils.py	69.04% <42.85%> (-4.45%)	⬇️
mmengine/utils/package_utils.py	47.05% <50.00%> (-2.95%)	⬇️
... and 7 more

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