Fix palette type.

[yaochaorui]

Traceback` (most recent call last):  File "tools/demo.py", line 21, in <module>
    model.show_result(img, result, out_file='demo_res/'+img)
  File 'mmsegmentation/mmseg/models/segmentors/base.py", line 247, in show_result
    assert palette.shape[0] == len(self.CLASSES)
AttributeError: 'list' object has no attribute 'shape'

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[codecov]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   81.41%   81.41%           
=======================================
  Files          71       71           
  Lines        3158     3158           
  Branches      493      493           
=======================================
  Hits         2571     2571           
  Misses        497      497           
  Partials       90       90           

Flag	Coverage Δ
#unittests	81.41% <0.00%> (ø)

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Impacted Files	Coverage Δ
mmseg/models/segmentors/base.py	45.37% <0.00%> (ø)

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[OpenMMLab-Assistant-007]

Hi!
@yaochaorui
First of all, we want to express our gratitude for your significant PR in the OpenMMLab project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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