[Fix] Fix mlp_ratio type in Swin Transformer

[MengzhangLI]

In more and more cases of Transformer models, it may use float mlp_ratio such as 0.25 or something else.

It would be simply realized by int( ).

[codecov]

Codecov Report

Merging #1274 (ece92a5) into master (b4314f9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1274   +/-   ##
=======================================
  Coverage   90.26%   90.26%           
=======================================
  Files         130      130           
  Lines        7553     7553           
  Branches     1257     1257           
=======================================
  Hits         6818     6818           
  Misses        528      528           
  Partials      207      207           

Flag	Coverage Δ
unittests	90.26% <ø> (ø)

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Impacted Files	Coverage Δ
mmseg/models/backbones/swin.py	83.62% <ø> (ø)

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