Add autocast support for einsum #10547

Job	Run time
get-torch-commit	1s
Build PyTorch/XLA / build	1h 8m 24s
TPU tests	0s
Build docs / build-docs	1m 42s
CPU tests / test (benchmark_tests)	8m 25s
CPU tests / test (python_tests, xla_op1)	17m 23s
CPU tests / test (python_tests, xla_op2)	6m 20s
CPU tests / test (python_tests, xla_op3)	8m 55s
CPU tests / test (python_tests, torch_mp_op)	13m 2s
CPU tests / test (cpp_tests, cpp_tests1)	4m 28s
CPU tests / test (cpp_tests, cpp_tests2)	5m 42s
	2h 14m 22s

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