Delete baseline pipeline classes

[angela97lin]

Closes #2175

Also fixes minor bug with getting input feature names for baseline / pipelines with just one component :)

To discuss:

• Still keep tests as automl tests? I decided to add some tests in automl to make sure that we're getting the right predictions.

[codecov]

Codecov Report

Merging #2202 (defdc8f) into main (9d108d7) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #2202     +/-   ##
=========================================
- Coverage   100.0%   100.0%   -0.0%     
=========================================
  Files         296      287      -9     
  Lines       24712    24451    -261     
=========================================
- Hits        24694    24433    -261     
  Misses         18       18             

Impacted Files	Coverage Δ
evalml/pipelines/__init__.py	100.0% <ø> (ø)
evalml/utils/gen_utils.py	99.6% <ø> (ø)
evalml/automl/automl_search.py	100.0% <100.0%> (ø)
evalml/pipelines/component_graph.py	100.0% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	99.7% <100.0%> (+0.1%)	⬆️
.../automl_tests/test_automl_search_classification.py	100.0% <100.0%> (ø)
...ests/automl_tests/test_automl_search_regression.py	100.0% <100.0%> (ø)
...s/prediction_explanations_tests/test_algorithms.py	99.2% <100.0%> (ø)

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[angela97lin]

Just separated out helpers to make logic of getting baseline abstracted from training baseline :) No functional change.

[bchen1116]

Nice changes! LGTM

[freddyaboulton]

Looks great @angela97lin ! Agree it's a good idea to add tests for the baseline predictions in automl.

[dsherry]

🚢 😁