Add Decipher post-training methods

[justjhong]

*TODO: merge trajectory methods PR into this PR (chunked for ease of review)

Adds the following downstream methods to the Decipher class.

• rotate_decipher_components
• compute_imputed_gene_expression
• compute_decipher_time
• compute_gene_patterns

This is the remaining functionality desired to transfer from the original implementation.
Note, this implementation takes a more scvi-tools-esque approach to each of the methods,
avoiding saving results to the anndata object directly.

cc @ANazaret

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 8 lines in your changes missing coverage. Please review.

Project coverage is 84.66%. Comparing base (77a1458) to head (84d8378).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scvi/external/decipher/utils/_utils.py	87.50%	6 Missing ⚠️
src/scvi/external/decipher/_model.py	90.90%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (77a1458) and HEAD (84d8378). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (77a1458)	HEAD (84d8378)
	13	3

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3091      +/-   ##
==========================================
- Coverage   90.09%   84.66%   -5.44%     
==========================================
  Files         178      180       +2     
  Lines       15118    15190      +72     
==========================================
- Hits        13621    12861     -760     
- Misses       1497     2329     +832     

Files with missing lines	Coverage Δ
src/scvi/external/decipher/utils/__init__.py	100.00% <100.00%> (ø)
src/scvi/external/decipher/_model.py	97.36% <90.90%> (-2.64%)	⬇️
src/scvi/external/decipher/utils/_utils.py	87.50% <87.50%> (ø)

... and 22 files with indirect coverage changes