Remove hard dependency on TensorFlow

[adriangb]

Closes #221

[codecov-commenter]

Codecov Report

Merging #237 (8bc28cc) into master (7c072fb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   98.90%   98.90%           
=======================================
  Files           6        6           
  Lines         728      733    +5     
=======================================
+ Hits          720      725    +5     
  Misses          8        8           

Impacted Files	Coverage Δ
scikeras/__init__.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit 8bc28cc at: https://www.adriangb.com/scikeras/refs/pull/237/merge/

[adriangb]

Nightly tests seem to be failing on all branches. I'll look into that separately.