Replace custom `attrdict` with `ml_collections` implementation.

[martinkim0]

Closes #1690.

[codecov[bot]]

Codecov Report

Base: 90.71% // Head: 90.84% // Increases project coverage by +0.12% :tada:

Coverage data is based on head (c7c34d2) compared to base (d09368d). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1696      +/-   ##
==========================================
+ Coverage   90.71%   90.84%   +0.12%     
==========================================
  Files         116      116              
  Lines        9791     9764      -27     
==========================================
- Hits         8882     8870      -12     
+ Misses        909      894      -15     

Impacted Files	Coverage Δ
scvi/data/_utils.py	94.21% <ø> (+7.09%)	:arrow_up:
scvi/module/_totalvae.py	88.19% <ø> (+0.64%)	:arrow_up:
scvi/data/_manager.py	98.19% <100.00%> (ø)
scvi/data/fields/_protein.py	98.21% <100.00%> (ø)
scvi/model/_totalvi.py	87.59% <100.00%> (ø)
scvi/utils/_attrdict.py	100.00% <100.00%> (+11.76%)	:arrow_up:
scvi/module/_multivae.py	82.07% <0.00%> (+0.22%)	:arrow_up:

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[martinkim0]

Seems that ml_collections does not allow ConfigDicts to be initialized with nested dictionaries where inner dictionaries have non-iterable types (e.g. ints) as keys (#13). This comes up as a problem when initializing ~scvi.model.TOTALVI, since "protein_batch_mask" has ints as keys.