Addition of `step_warmup` #351

Job	Run time
test (1.6, ubuntu-latest, x86, 1)	7m 3s
test (1.6, ubuntu-latest, x64, 1)	5m 4s
test (1.6, macOS-latest, x64, 1)	5m 37s
test (1.6, windows-latest, x64, 1)	14m 44s
test (1, ubuntu-latest, x86, 1)	30m 40s
test (1, ubuntu-latest, x86, 2)	32m 4s
test (1, ubuntu-latest, x64, 1)	5m 34s
test (1, ubuntu-latest, x64, 2)	4m 47s
test (1, macOS-latest, x64, 1)	6m 3s
test (1, macOS-latest, x64, 2)	6m 16s
test (1, windows-latest, x64, 1)	5m 45s
test (1, windows-latest, x64, 2)	5m 42s
test (nightly, ubuntu-latest, x86, 1)	9m 0s
test (nightly, ubuntu-latest, x86, 2)	7m 36s
test (nightly, ubuntu-latest, x64, 1)	5m 44s
test (nightly, ubuntu-latest, x64, 2)	5m 47s
test (nightly, macOS-latest, x64, 1)	7m 23s
test (nightly, macOS-latest, x64, 2)	6m 33s
test (nightly, windows-latest, x64, 1)	6m 38s
test (nightly, windows-latest, x64, 2)	6m 46s
	3h 4m 46s

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