Slice and keep dim for easy deployment of vit

[RunningLeon]

Motivation

Operations on weight is not good for tracing and exporting to ONNX.

Modification

Use slice to keep dim in mmseg/models/backbones/vit.py

BC-breaking (Optional)

None

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 MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1642 (056a906) into master (46326f6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1642      +/-   ##
==========================================
- Coverage   90.25%   90.25%   -0.01%     
==========================================
  Files         142      142              
  Lines        8477     8476       -1     
  Branches     1428     1428              
==========================================
- Hits         7651     7650       -1     
  Misses        586      586              
  Partials      240      240              

Flag	Coverage Δ
unittests	90.25% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmseg/models/backbones/vit.py	90.85% <100.00%> (-0.06%)	⬇️

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