Fix: Extract some methods from FCLayers for easier inheritance

[moinfar]

Add _build_layer, _is_linear_layer, _apply_batch_norm, and _apply_layer as overridable hooks so subclasses (e.g. SplitFCLayers in DRVI) can customise layer construction and dispatch without duplicating the forward loop and scArches logic. Store constructor args as instance attributes so _build_layer can reference them.

Also, fix set_online_update_hooks to use [..., dims_to_update] instead of [:, dims_to_update] so the gradient mask works on 3-D weight tensors (as in DRVI decoder).

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (ed5485f) to head (99cd0ac).

Files with missing lines	Patch %	Lines
src/scvi/nn/_base_components.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3880   +/-   ##
=======================================
  Coverage   88.54%   88.54%           
=======================================
  Files         231      231           
  Lines       23050    23064   +14     
=======================================
+ Hits        20409    20423   +14     
  Misses       2641     2641           

Flag	Coverage Δ
cuda	74.28% <96.29%> (+0.04%)	⬆️
integration	74.25% <96.29%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/scvi/nn/_base_components.py	94.79% <96.29%> (+0.26%)	⬆️

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