[Bug] Fix a bug in base distiller and add yolox distill example

[pppppM]

Motivation

As the title

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 #192 (8d962ff) into master (3cc359e) will increase coverage by 1.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   63.95%   65.88%   +1.93%     
==========================================
  Files          93       93              
  Lines        3462     3462              
  Branches      640      640              
==========================================
+ Hits         2214     2281      +67     
+ Misses       1136     1077      -59     
+ Partials      112      104       -8     

Flag	Coverage Δ
unittests	65.85% <ø> (+1.90%)	⬆️

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

Impacted Files	Coverage Δ
mmrazor/models/distillers/base.py	42.25% <ø> (ø)
mmrazor/models/pruners/structure_pruning.py	91.93% <0.00%> (+2.30%)	⬆️
mmrazor/utils/misc.py	100.00% <0.00%> (+4.76%)	⬆️
mmrazor/models/algorithms/autoslim.py	71.31% <0.00%> (+14.72%)	⬆️
mmrazor/utils/setup_env.py	100.00% <0.00%> (+27.27%)	⬆️
mmrazor/models/pruners/ratio_pruning.py	90.41% <0.00%> (+30.13%)	⬆️
mmrazor/models/pruners/utils/switchable_bn.py	100.00% <0.00%> (+69.23%)	⬆️

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