Add SaveCheckpoint callback for checkpointing

[martinkim0]

• Closes Add changes to make saving intermediate checkpoints easier #2264
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest docs/release_notes/index.md file if fixing a bug or adding a new feature.
• If the changes are patches for a version, I have added the on-merge: backport to x.x.x label.

To do

• Document what arguments are unsupported

[martinkim0]

Could be useful to add a post-training cleanup method in BaseModelClass that handles setting attributes such as is_trained_ once TrainRunner completes, since there is some redundant code upon saving a checkpoint.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (6fe4dda) 89.26% compared to head (7a7ef56) 89.29%.

Files	Patch %	Lines
scvi/train/_callbacks.py	90.24%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2317      +/-   ##
==========================================
+ Coverage   89.26%   89.29%   +0.03%     
==========================================
  Files         145      145              
  Lines       11827    11872      +45     
==========================================
+ Hits        10557    10601      +44     
- Misses       1270     1271       +1     

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