Changed per-pipeline timings to store as a float

[christopherbunn]

Resolves #4162

[codecov]

Codecov Report

Merging #4160 (6d4c169) into main (7b2b5d8) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #4160     +/-   ##
=======================================
- Coverage   99.7%   99.7%   -0.0%     
=======================================
  Files        349     349             
  Lines      37781   37777      -4     
=======================================
- Hits       37664   37660      -4     
  Misses       117     117             

Impacted Files	Coverage Δ
evalml/utils/logger.py	100.0% <ø> (ø)
evalml/automl/automl_search.py	99.6% <100.0%> (-<0.1%)	⬇️
evalml/tests/automl_tests/test_automl.py	99.6% <100.0%> (ø)
evalml/tests/utils_tests/test_logger.py	100.0% <100.0%> (ø)

[jeremyliweishih]

LGTM

[eccabay]

Looks great! One request before approval - could you replace elapsed_time = time_elapsed(self.progress.start_time) (line 1170) as well? It'd keep things more consistent.