[Doc] SUNRGBD dataset doc update

[yezhen17]

Updated SUNRGBD dataset doc to stress the additional requirements for training ImVoteNet; also changed the image path in SUNRGBD data loading for convenience.

[codecov]

Codecov Report

Merging #391 (c7e98f8) into master (dabf0a2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head c7e98f8 differs from pull request most recent head 4125c67. Consider uploading reports for the commit 4125c67 to get more accurate results

@@           Coverage Diff           @@
##           master     #391   +/-   ##
=======================================
  Coverage   49.71%   49.71%           
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  Files         181      181           
  Lines       12451    12450    -1     
  Branches     1951     1950    -1     
=======================================
  Hits         6190     6190           
  Misses       5871     5871           
+ Partials      390      389    -1     

Flag	Coverage Δ
unittests	49.71% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmdet3d/datasets/sunrgbd_dataset.py	72.60% <100.00%> (+0.98%)	⬆️
mmdet3d/models/voxel_encoders/voxel_encoder.py	54.40% <100.00%> (ø)

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