Add basic device APIs to the top-level `torch_xla` module. #5003

Job	Run time
Build XLA / build	1h 0m 49s
Build XLA / build	1h 0m 49s
Build & publish docs	0s
Build & publish docs	0s
GPU tests / test (benchmark_tests)	13m 22s
GPU tests / test (benchmark_tests)	13m 22s
CPU tests / test (benchmark_tests)	11m 28s
CPU tests / test (benchmark_tests)	11m 28s
GPU tests / test (cpp_tests1)	15m 54s
GPU tests / test (cpp_tests1)	19m 49s
CPU tests / test (cpp_tests1)	14m 2s
CPU tests / test (cpp_tests1)	16m 4s
CPU tests / test (cpp_tests2)	12m 48s
CPU tests / test (cpp_tests2)	19m 19s
GPU tests / test (cpp_tests2)	12m 33s
GPU tests / test (cpp_tests2)	30m 20s
CPU tests / test (python_tests, xla_op1)	21m 46s
CPU tests / test (python_tests, xla_op1)	21m 46s
GPU tests / test (python_tests, xla_op1)	2h 17m 16s
GPU tests / test (python_tests, xla_op1)	2h 17m 16s
CPU tests / test (python_tests, xla_op2)	11m 15s
CPU tests / test (python_tests, xla_op2)	11m 15s
GPU tests / test (python_tests, xla_op2)	50m 14s
GPU tests / test (python_tests, xla_op2)	50m 14s
CPU tests / test (python_tests, xla_op3)	12m 48s
CPU tests / test (python_tests, xla_op3)	12m 48s
GPU tests / test (python_tests, xla_op3)	53m 45s
GPU tests / test (python_tests, xla_op3)	53m 45s
CPU tests / test (python_tests, torch_mp_op)	26m 21s
CPU tests / test (python_tests, torch_mp_op)	26m 21s
GPU tests / test (python_tests, torch_mp_op)	2h 32m 57s
GPU tests / test (python_tests, torch_mp_op)	2h 32m 57s
	20h 44m 51s

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