Merge pull request #89 from jiangyi15/cached_amp #427

Job	Run time
conda install (ubuntu-20.04)	5m 51s
Build documentation	2m 39s
Unit tests (ubuntu-20.04, 3.7)	15m 42s
conda install latest tensorflow (ubuntu-20.04)	13m 52s
Unit tests (macos-11, 3.7)	22m 1s
Unit tests (ubuntu-20.04, 3.6)	17m 44s
Unit tests (ubuntu-20.04, 3.8)	18m 54s
Test uproot3 (ubuntu-20.04, 3.7)	21m 18s
Style checks	2m 44s
Coverage (ubuntu-20.04, 3.7)	24s
	2h 1m 9s

Provide feedback