`cummax`: fix 0-sized dimension reduction. #10932

Job	Run time
get-torch-commit	3s
get-torch-commit	3s
Build PyTorch/XLA / build	19m 12s
Build PyTorch/XLA / build	33m 34s
TPU tests	0s
CPU tests	0s
Build docs	0s
Build docs / build-docs	1m 59s
TPU tests / tpu-test	53m 4s
CPU tests / test (benchmark_tests)	8m 37s
CPU tests / test (python_tests, xla_op1)	18m 5s
CPU tests / test (python_tests, xla_op2)	6m 41s
CPU tests / test (python_tests, xla_op3)	9m 54s
CPU tests / test (python_tests, torch_mp_op)	13m 37s
CPU tests / test (cpp_tests, cpp_tests1)	5m 32s
CPU tests / test (cpp_tests, cpp_tests2)	5m 54s
	2h 56m 15s

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