Refactor datapipes

[talmo]

Owing to pytorch/data#1196, this PR removes the dependency on the standalone torchdata library in favor of the built-in torch.utils.data.datapipes implementation.

It also adds the normalizer block and refactors some of the existing classes and tests.

[codecov]

Codecov Report

Merging #9 (fc87457) into main (e579925) will increase coverage by 1.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   96.07%   97.43%   +1.35%     
==========================================
  Files           4        6       +2     
  Lines          51       78      +27     
==========================================
+ Hits           49       76      +27     
  Misses          2        2              

Impacted Files	Coverage Δ
sleap_nn/data/augmentation.py	100.00% <100.00%> (ø)
sleap_nn/data/instance_centroids.py	100.00% <100.00%> (ø)
sleap_nn/data/normalization.py	100.00% <100.00%> (ø)
sleap_nn/data/providers.py	100.00% <100.00%> (ø)

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