[Fix] Fix the bug that occurred when whitespace is in a new dataset path

[pvys]

When reorganizing a new dataset to the existing format, if whitespace is included in the path of that data, it'll be split into more than two values. It'll occur an error at the 1100th line for filename, gt_label in self.samples:
e.g.
For an annotation list like

ILSVRC  2012_val_00000001.JPEG 65
ILSVRC  2012_val_00000002.JPEG 970
ILSVRC  2012_val_00000003.JPEG 230
ILSVRC  2012_val_00000004.JPEG 809
ILSVRC  2012_val_00000005.JPEG 516

/mmclassification/mmcls/datasets/imagenet.py in load_annotations(self)
   1099         data_infos = []
-> 1100         for filename, gt_label in self.samples:
   1101             info = {'img_prefix': self.data_prefix}

ValueError: too many values to unpack (expected 2)

This'll fix the issue.

[CLAassistant]

All committers have signed the CLA.

[mzr1996]

We will check it recently. Thanks for your contribution!

[codecov]

Codecov Report

Merging #370 (92b9564) into master (c92eefb) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #370   +/-   ##
=======================================
  Coverage   76.02%   76.02%           
=======================================
  Files          96       96           
  Lines        5114     5114           
  Branches      849      849           
=======================================
  Hits         3888     3888           
  Misses       1108     1108           
  Partials      118      118           

Flag	Coverage Δ
unittests	76.02% <0.00%> (ø)

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

Impacted Files	Coverage Δ
mmcls/datasets/imagenet.py	24.48% <0.00%> (ø)

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[mzr1996]

LGTM

[OpenMMLab-Coodinator]

Hi @pvys ！First of all, we want to express our gratitude for your significant PR in this 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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