[Improvement] Decouple dependency

[fpshuang]

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Motivation

Decouple dependencies.

Modification

add get_placeholder function

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
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

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 tests 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

Merging #254 (116cc04) into dev-1.x (5d9fcd8) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           dev-1.x    #254      +/-   ##
==========================================
- Coverage     0.41%   0.41%   -0.01%     
==========================================
  Files          172     173       +1     
  Lines         6931    6972      +41     
  Branches      1115    1116       +1     
==========================================
  Hits            29      29              
- Misses        6897    6938      +41     
  Partials         5       5              

Flag	Coverage Δ
unittests	0.41% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...ls/architectures/backbones/searchable_mobilenet.py	0.00% <0.00%> (ø)
...rchitectures/backbones/searchable_shufflenet_v2.py	0.00% <0.00%> (ø)
...or/models/architectures/heads/darts_subnet_head.py	0.00% <0.00%> (ø)
mmrazor/models/losses/kd_soft_ce_loss.py	0.00% <0.00%> (ø)
mmrazor/models/ops/efficientnet_series.py	0.00% <0.00%> (ø)
mmrazor/models/ops/mobilenet_series.py	0.00% <0.00%> (ø)
mmrazor/models/ops/shufflenet_series.py	0.00% <0.00%> (ø)
...oss_calculator/image_classifier_loss_calculator.py	0.00% <0.00%> (ø)
mmrazor/utils/__init__.py	0.00% <0.00%> (ø)
mmrazor/utils/placeholder.py	0.00% <0.00%> (ø)

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