Fixed IndexError when ensembling = True but number of pipelines to search over is 1

[angela97lin]

Closes #1387

• Fixes IndexError when number of pipelines to search over == 1
• Adds a warning that ensembling will not run if num of pipelines == 1

[codecov]

Codecov Report

Merging #1397 (8decbbe) into main (cf50801) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #1397     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         214      214             
  Lines       14073    14133     +60     
=========================================
+ Hits        14066    14126     +60     
  Misses          7        7             

Impacted Files	Coverage Δ
...lml/automl/automl_algorithm/iterative_algorithm.py	100.0% <100.0%> (ø)
evalml/automl/automl_search.py	99.7% <100.0%> (+0.1%)	⬆️
evalml/tests/automl_tests/test_automl.py	100.0% <100.0%> (ø)
...lml/tests/automl_tests/test_iterative_algorithm.py	100.0% <100.0%> (ø)

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[freddyaboulton]

Looks great @angela97lin ! I left some minor comments/questions/suggestions.

[bchen1116]

LGTM!