[Fix] Remove unexist arg in docstring

[KangBK0120]

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Motivation

Might be trivial but it may be confusing to someone.

Modification

It seems that the loss_type arg in DiceLoss is not used anywhere but it exists in the docstring and docs.

[CLAassistant]

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[MengzhangLI]

Hi, thanks for your pr.

The 'loss_type' has been removed in this PR, the related docstring should also be removed.

[codecov]

Codecov Report

Merging #1106 (3cd8c6e) into master (2918220) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1106   +/-   ##
=======================================
  Coverage   89.53%   89.53%           
=======================================
  Files         122      122           
  Lines        6931     6931           
  Branches     1156     1156           
=======================================
  Hits         6206     6206           
  Misses        519      519           
  Partials      206      206           

Flag	Coverage Δ
unittests	89.53% <ø> (ø)

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

Impacted Files	Coverage Δ
mmseg/models/losses/dice_loss.py	100.00% <ø> (ø)

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