Add support for adding tuple outputs in the configure() method

[abadams]

We also weren't asserting that the names used in add_output weren't already taken, which resulted in some confusing internal errors when I made that mistake.

Fixes #8643

[ongjunjie]

Thanks for the patch. I want to point out that the new set_type function opens the possibility of setting inputs to Tuple types, which is in conflict with the comment here

Halide/src/Generator.h

Lines 1433 to 1435 in 16fde03

	* A GIOBase can have multiple Types, in which case it represents a Tuple.
	* (Note that Tuples are currently only supported for GeneratorOutput, but
	* it is likely that GeneratorInput will be extended to support Tuple as well.)

I'm not familiar enough with the compilation pipeline to say if it'll lead to undefined behaviour. It might be better to declare the new set_type signature as protected and only expose it as public in the output class.

[abadams]

Good call, I'll move it to the generator output subclass.

[alexreinking]

Looks like a fuzz test failure?

Error: Rewrite changed type: uint8x4((uint16x4)halving_add((uint16x4)widening_add((uint8x4)t4653, x4((uint8)1)), x4((uint16)88))) -> (uint16x4)rounding_halving_add(uint16x4((uint8x4)t4653), x4((uint16)88))

[abadams]

Fuzz failure fixed by #8674

[abadams]

Failures unrelated