Follow-up's to #681

[ascillitoe]

Resolves #683

Progress tracker:

• Item 1: replace cfg = self.config.copy() with deepcopy's.
• Item 2: replace kernel_a: Union[nn.Module, str] = 'rbf''s with Literal's in the kernel submodules.
• Item 3: refactor if statements in saving subpackage. - decided just to replace else's with elif's

[codecov-commenter]

Codecov Report

Merging #723 (0502d81) into master (71d41ef) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head 0502d81 differs from pull request most recent head 5ad9237. Consider uploading reports for the commit 5ad9237 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
- Coverage   80.15%   80.07%   -0.09%     
==========================================
  Files         133      133              
  Lines        9177     9140      -37     
==========================================
- Hits         7356     7319      -37     
  Misses       1821     1821              

Flag	Coverage Δ
macos-latest-3.10	?
ubuntu-latest-3.10	?
ubuntu-latest-3.7	79.95% <100.00%> (+<0.01%)	⬆️
ubuntu-latest-3.8	?
ubuntu-latest-3.9	80.00% <100.00%> (+<0.01%)	⬆️
windows-latest-3.9	?

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Impacted Files	Coverage Δ
alibi_detect/utils/pytorch/kernels.py	97.70% <100.00%> (+0.05%)	⬆️
alibi_detect/utils/tensorflow/kernels.py	97.72% <100.00%> (+0.05%)	⬆️
alibi_detect/utils/frameworks.py	88.88% <0.00%> (-3.71%)	⬇️
alibi_detect/saving/validators.py	88.46% <0.00%> (-1.93%)	⬇️
alibi_detect/utils/missing_optional_dependency.py	94.28% <0.00%> (-0.16%)	⬇️
alibi_detect/datasets.py	68.55% <0.00%> (-0.14%)	⬇️
alibi_detect/saving/schemas.py	98.69% <0.00%> (-0.09%)	⬇️
alibi_detect/utils/fetching/fetching.py	0.00% <0.00%> (ø)
alibi_detect/cd/base.py	91.19% <0.00%> (+0.21%)	⬆️
alibi_detect/od/vae.py	90.36% <0.00%> (+1.07%)	⬆️