Make batch argument optionnal in voxel_grid / update GridSampling documentation

[QuanticDisaster]

This PR does the following :

• adds documentation on return nature of GridSampling
• removes the mandatory argument 'batch' for voxel_grid, defaulting it to None

[codecov-commenter]

Codecov Report

Merging #3533 (a1416f5) into master (ceba827) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3533   +/-   ##
=======================================
  Coverage   81.83%   81.84%           
=======================================
  Files         292      292           
  Lines       14574    14576    +2     
=======================================
+ Hits        11927    11929    +2     
  Misses       2647     2647           

Impacted Files	Coverage Δ
torch_geometric/nn/pool/voxel_grid.py	100.00% <100.00%> (ø)
torch_geometric/transforms/grid_sampling.py	96.77% <100.00%> (ø)

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[rusty1s]

This is great. Thank you!