Add CI overview to `ci.md` (#7015) #8073

Job	Run time
Build PyTorch/XLA / build	22m 55s
Build XLA CUDA plugin / build	4m 40s
GPU tests / test (benchmark_tests)	5m 25s
TPU tests / tpu-test	14m 29s
GPU tests / test (python_tests, xla_op1)	27m 32s
CPU tests / test (benchmark_tests)	2m 3s
GPU tests / test (python_tests, xla_op2)	5m 18s
Build docs / build-docs	1m 33s
CPU tests / test (python_tests, xla_op1)	10m 59s
GPU tests / test (python_tests, xla_op3)	43m 27s
CPU tests / test (python_tests, xla_op2)	1m 43s
GPU tests / test (python_tests, torch_mp_op)	47m 23s
CPU tests / test (python_tests, xla_op3)	4m 1s
GPU tests / test (cpp_tests, cpp_tests1)	7m 47s
CPU tests / test (python_tests, torch_mp_op)	15m 1s
GPU tests / test (cpp_tests, cpp_tests2)	15m 30s
CPU tests / test (cpp_tests, cpp_tests1)	3m 2s
CPU tests / test (cpp_tests, cpp_tests2)	3m 25s
	3h 56m 13s

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