[Train] Support FSDP2 checkpoint resume #435

Job	Run time
run_tests / checkout_and_config	9s
run_tests / unit_tests (a100) / unit-a100	1m 15s
run_tests / functional_tests_hetero_train	0s
run_tests / functional_tests_serve	0s
run_tests / functional_tests_inference	0s
run_tests / functional_tests_train	0s
run_tests / all_tests_complete	3s
all_tests	2s
	1m 29s

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