Replace `dask_ml.wrappers.ParallelPostFit` with custom `ParallelPostFit` class

[sarahyurick]

As part of our initiative to move away from Dask-ML, I've migrated some code from Dask-ML into Dask-SQL to support ParallelPostFit.

[codecov-commenter]

Codecov Report

Merging #832 (ef087db) into main (d9fc2c1) will decrease coverage by 2.10%. The diff coverage is 30.16%.

@@            Coverage Diff             @@
##             main     #832      +/-   ##
==========================================
- Coverage   77.44%   75.33%   -2.11%     
==========================================
  Files          71       72       +1     
  Lines        3600     3779     +179     
  Branches      634      674      +40     
==========================================
+ Hits         2788     2847      +59     
- Misses        685      795     +110     
- Partials      127      137      +10     

Impacted Files	Coverage Δ
dask_sql/physical/rel/custom/wrappers.py	28.07% <28.07%> (ø)
dask_sql/physical/rel/custom/create_model.py	88.70% <66.66%> (-2.82%)	:arrow_down:
dask_sql/physical/rel/custom/create_experiment.py	96.15% <100.00%> (-0.10%)	:arrow_down:
dask_sql/_version.py	35.31% <0.00%> (+2.74%)	:arrow_up:

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[sarahyurick]

Let me know if you think any tests from Dask-ML's tests/test_parallel_post_fit.py or from https://github.com/dask/dask-ml/pull/912 should be added.