Add autocast support for XlaPatchedLinear #10558

Job	Run time
get-torch-commit	3s
Build PyTorch/XLA / build	1h 8m 35s
TPU tests	0s
Build docs / build-docs	1m 40s
CPU tests / test (benchmark_tests)	8m 10s
CPU tests / test (python_tests, xla_op1)	17m 7s
CPU tests / test (python_tests, xla_op2)	5m 9s
CPU tests / test (python_tests, xla_op3)	7m 44s
CPU tests / test (python_tests, torch_mp_op)	11m 48s
CPU tests / test (cpp_tests, cpp_tests1)	5m 3s
CPU tests / test (cpp_tests, cpp_tests2)	5m 21s
	2h 10m 40s

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