Move default_parameters to ComponentGraph from PipelineBase

[angela97lin]

Closes #2167

[codecov]

Codecov Report

Merging #2307 (de77ea8) into main (f01a4d0) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #2307   +/-   ##
=====================================
  Coverage   99.9%   99.9%           
=====================================
  Files        281     281           
  Lines      24608   24607    -1     
=====================================
- Hits       24579   24578    -1     
  Misses        29      29           

Impacted Files	Coverage Δ
evalml/pipelines/pipeline_base.py	99.7% <ø> (-<0.1%)	⬇️
evalml/pipelines/component_graph.py	100.0% <100.0%> (ø)
...lml/tests/automl_tests/test_iterative_algorithm.py	100.0% <100.0%> (ø)
evalml/tests/pipeline_tests/test_pipelines.py	99.9% <100.0%> (ø)

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[bchen1116]

Nice!

[freddyaboulton]

@angela97lin Looks good! I have a question about the release notes though.

[freddyaboulton]

I think users would need to do pipeline._component_graph. How come users need to access a private attribute to get the default parameters now? I think I missed the context for this change 🙈 Is there a plan to make pipeline.component_graph be a reference to the ComponentGraph as opposed to list/dict representation?

[angela97lin]

Yup, you're right!

Filed #2331, gonna work on it in a separate branch c:
(I believe its because we relied on the input component graph aka component_graph to determine if linear but also wanted to store _component_graph aka the underlying graph structure.)