Add types to embedding scripts

[agpeshal]

#1390

[guarin]

This is awesome, thanks a lot for the PR!

I made some minor changes:

• Replace calls to cast as it sometimes hides actual typing errors
• Make sort_items_by_keys generic
• Use torch.optim._LRScheduler instead of torch.optim.LRScheduler for backwards compatibility

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% 🎉

Comparison is base (f5a3b2e) 85.26% compared to head (90d293a) 85.29%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1407      +/-   ##
==========================================
+ Coverage   85.26%   85.29%   +0.03%     
==========================================
  Files         130      130              
  Lines        5456     5469      +13     
==========================================
+ Hits         4652     4665      +13     
  Misses        804      804              

Files Changed	Coverage Δ
lightly/embedding/_base.py	96.49% <100.00%> (+0.74%)	⬆️
lightly/embedding/callbacks.py	100.00% <100.00%> (ø)
lightly/embedding/embedding.py	100.00% <100.00%> (ø)
lightly/utils/reordering.py	100.00% <100.00%> (ø)

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