Redesign Mask

[chengchingwen]

The goal of the redesign is to support:

1. better type hierarchy for dispatch. This helps the function that can only accept sequence masks to work with combined masks. https://github.com/chengchingwen/Transformers.jl/blob/91a3fe00bad5bb9ebff35b61356c3d52ad3efba3/src/loss.jl#L29-L31
2. make it easier to use the mask in GPU kernel (for #23)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.10843% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 74.52%. Comparing base (40922f8) to head (3af9063). Report is 2 commits behind head on master.

:exclamation: Current head 3af9063 differs from pull request most recent head 297cc23

Please upload reports for the commit 297cc23 to get more accurate results.

Files	Patch %	Lines
src/mask/indexer.jl	71.79%	11 Missing :warning:
src/mask/mask.jl	66.66%	7 Missing :warning:
src/mask/broadcast.jl	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   74.51%   74.52%   +0.01%     
==========================================
  Files          30       30              
  Lines        2052     2057       +5     
==========================================
+ Hits         1529     1533       +4     
- Misses        523      524       +1     

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