Add `xla.step` context manager #8209

Job	Run time
Build PyTorch/XLA / build	29m 15s
Build XLA CUDA plugin / build	5m 40s
TPU tests	0s
CPU tests / test (benchmark_tests)	1m 27s
CPU tests / test (python_tests, xla_op1)	10m 51s
CPU tests / test (python_tests, xla_op2)	1m 44s
CPU tests / test (python_tests, xla_op3)	3m 8s
Build docs / build-docs	1m 29s
CPU tests / test (python_tests, torch_mp_op)	14m 52s
CPU tests / test (cpp_tests, cpp_tests1)	2m 58s
CPU tests / test (cpp_tests, cpp_tests2)	3m 20s
GPU tests / test (benchmark_tests)	5m 31s
GPU tests / test (python_tests, xla_op1)	30m 22s
GPU tests / test (python_tests, xla_op2)	5m 28s
GPU tests / test (python_tests, xla_op3)	8m 36s
GPU tests / test (python_tests, torch_mp_op)	48m 5s
GPU tests / test (cpp_tests, cpp_tests1)	8m 0s
GPU tests / test (cpp_tests, cpp_tests2)	15m 44s
	3h 16m 30s

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