[Fix] Fix EfficientMultiheadAttention in SegFormer

[Junjun2016]

Motivation

The latest MMCV(v1.3.17) include a commit open-mmlab/mmcv#1418, which break the compatibility of EfficientMultiheadAttention in MiT.

Modification

Change the forward of EfficientMultiheadAttention and add a legacy_forward.

Handle the mmcv version by this:

  # handle the BC-breaking from https://github.com/open-mmlab/mmcv/pull/1418 # noqa
  from mmseg import mmcv_version, digit_version
  if mmcv_version < digit_version('1.3.17'):
      self.forward = self.legacy_forward

Related PR open-mmlab/mmdetection#6488.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
No

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.55%. Comparing base (7a1c9a5) to head (5aa97f7).
Report is 302 commits behind head on master.

Files	Patch %	Lines
mmseg/models/backbones/mit.py	76.19%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
- Coverage   89.76%   89.55%   -0.21%     
==========================================
  Files         119      119              
  Lines        6605     6626      +21     
  Branches     1029     1034       +5     
==========================================
+ Hits         5929     5934       +5     
- Misses        475      488      +13     
- Partials      201      204       +3     

Flag	Coverage Δ
unittests	89.55% <76.19%> (-0.21%)	⬇️

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