[feat][minor] Simplicial embeddings

[blefaudeux]

What does this PR do?

Follow up to #259, adds simplicial embeddings + option to add it at the bottom of an encoder. I actually don't believe that it's super important (it was kind of easy to add manually beforehand), but it does not take much space (orthogonal to the rest), and I would be curious to see how that affects things like openclip and friends.

I've done a dummy test on the ViT Cifar training, it does not meaningfully affects the score (in either way), which is not too surprising (the linear probe afterwards could already recover from "misaligned" dimensions given the strong supervised signal, which is what simplicial embeddings could improve)

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PR review

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[codecov-commenter]

Codecov Report

Merging #261 (a4d3cec) into main (23aaa58) will decrease coverage by 0.03%.
The diff coverage is 89.74%.

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
- Coverage   92.83%   92.80%   -0.04%     
==========================================
  Files          60       61       +1     
  Lines        3324     3363      +39     
==========================================
+ Hits         3086     3121      +35     
- Misses        238      242       +4     

Flag	Coverage Δ
Python	92.80% <89.74%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
xformers/components/simplicial_embedding.py	87.09% <87.09%> (ø)
xformers/factory/block_factory.py	93.30% <100.00%> (+0.26%)	⬆️

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[dianaml0]

Nice!! Just had one comment