Replace indexing with tf.gather in MMDDriftOnlineTF._update_state

[ascillitoe]

Fixes #706

[codecov-commenter]

Codecov Report

Merging #710 (7ce6476) into master (f0b57b4) will increase coverage by 3.38%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #710      +/-   ##
==========================================
+ Coverage   76.65%   80.04%   +3.38%     
==========================================
  Files         133      133              
  Lines        9177     9177              
==========================================
+ Hits         7035     7346     +311     
+ Misses       2142     1831     -311     

Flag	Coverage Δ
macos-latest-3.10	?
ubuntu-latest-3.10	80.04% <100.00%> (?)
ubuntu-latest-3.9	79.99% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
alibi_detect/cd/tensorflow/mmd_online.py	98.48% <100.00%> (ø)
alibi_detect/utils/frameworks.py	88.88% <0.00%> (ø)
alibi_detect/saving/loading.py	93.71% <0.00%> (+0.48%)	⬆️
alibi_detect/cd/base.py	90.97% <0.00%> (+1.66%)	⬆️
alibi_detect/cd/learned_kernel.py	100.00% <0.00%> (+4.34%)	⬆️
alibi_detect/cd/mmd.py	97.72% <0.00%> (+6.81%)	⬆️
alibi_detect/saving/registry.py	100.00% <0.00%> (+10.71%)	⬆️
alibi_detect/saving/_keops/loading.py	88.23% <0.00%> (+76.47%)	⬆️
alibi_detect/cd/keops/learned_kernel.py	94.16% <0.00%> (+94.16%)	⬆️
alibi_detect/utils/keops/kernels.py	98.01% <0.00%> (+97.02%)	⬆️
... and 1 more

[mauicv]

LGTM!