Add support for TOKEN type to CPU/GPU backends.

TOKENs will be used for ordering side-effecting operations. They are not materialized but can be contained in tuples and flow into and out of computations. This CL adds a trivial representation for the cpu and gpu backends to support TOKENs and modifies copy insertion to avoid making copies of tokens.

This also adds a Literal TOKEN which is required for the interpreter backend.

PiperOrigin-RevId: 200623120