[Fix] Fix LoadMultiViewImageFromFiles to be compatible with DefaultFormatBundle

[Wuziyi616]

Fix issue #227. LoadMultiViewImageFromFiles is currently not used in the codebase. I simply unravel the stacked multi-view img to list because DefaultFormatBundle requires so (see here).

This solution is better than PR #229 I think.

[codecov]

Codecov Report

Merging #611 (3e62814) into master (d5bddd2) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
+ Coverage   50.95%   51.06%   +0.11%     
==========================================
  Files         197      197              
  Lines       15076    15059      -17     
  Branches     2452     2445       -7     
==========================================
+ Hits         7682     7690       +8     
+ Misses       6887     6865      -22     
+ Partials      507      504       -3     

Flag	Coverage Δ
unittests	51.06% <100.00%> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
mmdet3d/datasets/pipelines/loading.py	90.61% <100.00%> (+5.57%)	⬆️
mmdet3d/datasets/scannet_dataset.py	87.85% <0.00%> (ø)
mmdet3d/datasets/custom_3d_seg.py	67.53% <0.00%> (+0.03%)	⬆️
mmdet3d/datasets/s3dis_dataset.py	94.66% <0.00%> (+3.65%)	⬆️
mmdet3d/datasets/pipelines/formating.py	60.00% <0.00%> (+4.00%)	⬆️

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