Add masked average pooling for pooling with segmentation masks (DetCon)

[liopeer]

Changes

• add masked pooling for batched and unbatched feature maps together with integer masks

How has it been tested

• validated against examples calculated by hand

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.22%. Comparing base (8781c33) to head (484cdc5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lightly/models/utils.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1739      +/-   ##
==========================================
+ Coverage   85.21%   85.22%   +0.01%     
==========================================
  Files         162      162              
  Lines        6708     6729      +21     
==========================================
+ Hits         5716     5735      +19     
- Misses        992      994       +2     

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[guarin]

This looks great! Left some comments regarding formatting and code structure but the logic looks correct

[guarin]

LGTM! I left one more comment but feel free to resolve and merge.