[XLA] Introduce variadic version of reduce.
[XLA] Introduce variadic version of reduce.

This defines the semantics, and adds parser and shape inference support. Since support is not plumbed through the rest of the compiler here, multi-output reduce is still rejected by the HLO verifier, and is not exposed through XlaBuilder.

PiperOrigin-RevId: 207148035

tensorflow/docs,tensorflow/docs,tensorflow/docs-l10n,tensorflow/docs