[Fix] Fix the requirements and lazy register mmcls models.

[mzr1996]

Motivation

The modelindex package is missing in the requirements.

Modification

• Add it in the requirements/runtime.txt.
• In the mmcls.apis package, avoid importing some packages and registering mmcls models directly. This modification can speed up import mmcls (from ~3.2s to ~0.7s)

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.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 0.02% // Head: 88.56% // Increases project coverage by +88.54% 🎉

Coverage data is based on head (fba8d58) compared to base (b8b31e9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff              @@
##           dev-1.x    #1275       +/-   ##
============================================
+ Coverage     0.02%   88.56%   +88.54%     
============================================
  Files          121      151       +30     
  Lines         8217    11818     +3601     
  Branches      1368     1895      +527     
============================================
+ Hits             2    10467    +10465     
+ Misses        8215     1048     -7167     
- Partials         0      303      +303     

Flag	Coverage Δ
unittests	88.56% <ø> (+88.54%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/datasets/transforms/compose.py
mmcls/models/retrievers/base.py	100.00% <0.00%> (ø)
mmcls/models/heads/margin_head.py	89.13% <0.00%> (ø)
mmcls/engine/hooks/retriever_hooks.py	72.72% <0.00%> (ø)
mmcls/models/retrievers/image2image.py	90.90% <0.00%> (ø)
mmcls/models/utils/layer_scale.py	86.66% <0.00%> (ø)
mmcls/engine/hooks/switch_recipe_hook.py	88.46% <0.00%> (ø)
mmcls/models/retrievers/__init__.py	100.00% <0.00%> (ø)
mmcls/models/heads/multi_label_csra_head.py	40.54% <0.00%> (ø)
mmcls/models/backbones/replknet.py	93.00% <0.00%> (ø)
... and 142 more

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[Ezra-Yu]

LGTM.