Implementation of kernel metrics in landmark space

[joanglaunes]

This reuses code from a previous PR (#883)

[codecov]

Codecov Report

Merging #1708 (c560f6a) into master (880d056) will increase coverage by 0.81%.
The diff coverage is 19.66%.

@@            Coverage Diff             @@
##           master    #1708      +/-   ##
==========================================
+ Coverage   90.52%   91.33%   +0.81%     
==========================================
  Files         124      130       +6     
  Lines       12786    13322     +536     
==========================================
+ Hits        11573    12166     +593     
+ Misses       1213     1156      -57     

Flag	Coverage Δ
autograd	87.21% <17.55%> (?)
numpy	87.22% <16.82%> (-0.60%)	⬇️
pytorch	79.83% <18.97%> (-0.54%)	⬇️

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Impacted Files	Coverage Δ
geomstats/geometry/landmarks.py	25.40% <16.82%> (-74.60%)	⬇️
geomstats/_backend/autograd/autodiff.py	85.92% <100.00%> (ø)
geomstats/_backend/pytorch/__init__.py	97.08% <100.00%> (+0.25%)	⬆️
geomstats/_backend/pytorch/autodiff.py	92.96% <100.00%> (+0.86%)	⬆️
geomstats/_backend/pytorch/linalg.py	85.40% <0.00%> (-1.12%)	⬇️
geomstats/_backend/autograd/linalg.py	95.35% <0.00%> (ø)
geomstats/_backend/autograd/_common.py	91.67% <0.00%> (ø)
geomstats/_backend/autograd/random.py	100.00% <0.00%> (ø)
geomstats/_backend/autograd/_dtype.py	95.24% <0.00%> (ø)
... and 13 more

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