Add GKE support and various usability improvements in CheckpointManag… #2809

Job	Run time
Build XLA / build	41m 48s
CPU tests / test (cpp_tests1)	12m 21s
CPU tests / test (cpp_tests2)	13m 15s
CPU tests / test (python_tests, xla_op1)	16m 38s
Collect CPU test coverage / test	51m 17s
GPU tests / test (cpp_tests1)	17m 45s
Build & publish docs / push-docs	8m 36s
CPU tests / test (python_tests, xla_op2)	9m 9s
GPU tests / test (cpp_tests2)	31m 4s
GPU tests / test (python_tests, xla_op1)	1h 17m 59s
CPU tests / test (python_tests, xla_op3)	9m 36s
GPU tests / test (python_tests, xla_op2)	2h 12m 28s
CPU tests / test (python, torch_mp_op)	14m 3s
GPU tests / test (python_tests, xla_op3)	51m 57s
GPU tests / test (python, torch_mp_op)	37m 5s
Collect GPU test coverage / test	3h 50m 4s
	12h 35m 5s

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