[Feature] Support DCN on CPU

[ckkelvinchan]

Motivation

Since mmcv supports DCN on CPU, we can now support DCN in EDVR and BasicVSR++

Modification

Modify the following files:

1. mmedit/models/backbones/sr_backbones/basicvsr_pp.py
2. tests/test_models/test_backbones/test_sr_backbones/test_basicvsr_plusplus.py
3. tests/test_models/test_backbones/test_sr_backbones/test_edvr_net.py

Note

@wangruohui there seems to be a problem in pre-commit hook. I need to modify .pre-commit-config.yaml to use pre-commit hook. Could you help have a look?

[codecov]

Codecov Report

Merging #735 (17e75f5) into master (7245818) will increase coverage by 0.75%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #735      +/-   ##
==========================================
+ Coverage   82.18%   82.93%   +0.75%     
==========================================
  Files         215      215              
  Lines       12125    12121       -4     
  Branches     1946     1945       -1     
==========================================
+ Hits         9965    10053      +88     
+ Misses       1853     1763      -90     
+ Partials      307      305       -2     

Flag	Coverage Δ
unittests	82.92% <60.00%> (+0.75%)	⬆️

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

Impacted Files	Coverage Δ
...medit/models/backbones/sr_backbones/basicvsr_pp.py	76.08% <60.00%> (+15.98%)	⬆️
mmedit/models/backbones/sr_backbones/edvr_net.py	98.36% <0.00%> (+33.15%)	⬆️

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[wangruohui]

Close #730