Add helper functions getDefaultXLAGenerator and createXLAGenerator to XLA random number generator #13879

Job	Run time
Check Code Changes / check_files	14s
Build PyTorch/XLA / build	45m 42s
report-no-code-changes	0s
TPU tests	0s
Build docs	0s
CPU tests	0s
torchprime tests	0s
	45m 56s

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