[Fix] Change the upper version of mmcv to 1.5.0

[MengzhangLI]

Related PR: open-mmlab/mmocr#628

MMCV released v1.4.0 lately so the downstream repos should modify their upper version of mmcv.

[codecov]

Codecov Report

Merging #1096 (24b910d) into master (62346e2) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
- Coverage   89.57%   89.51%   -0.07%     
==========================================
  Files         120      120              
  Lines        6717     6678      -39     
  Branches     1122     1122              
==========================================
- Hits         6017     5978      -39     
  Misses        496      496              
  Partials      204      204              

Flag	Coverage Δ
unittests	89.51% <100.00%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
mmseg/__init__.py	90.32% <100.00%> (ø)
mmseg/models/segmentors/base.py	58.65% <0.00%> (-3.30%)	⬇️
mmseg/models/necks/fpn.py	54.54% <0.00%> (-0.59%)	⬇️
mmseg/models/decode_heads/cascade_decode_head.py	92.30% <0.00%> (-0.55%)	⬇️
mmseg/ops/encoding.py	91.66% <0.00%> (-0.44%)	⬇️
mmseg/models/losses/cross_entropy_loss.py	79.10% <0.00%> (-0.31%)	⬇️
mmseg/models/backbones/vit.py	89.44% <0.00%> (-0.26%)	⬇️
mmseg/models/backbones/resnest.py	83.72% <0.00%> (-0.25%)	⬇️
mmseg/datasets/cityscapes.py	85.00% <0.00%> (-0.19%)	⬇️
mmseg/models/decode_heads/decode_head.py	92.47% <0.00%> (-0.16%)	⬇️
... and 8 more

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