Change infeed and outfeed to take and produce tokens.

Tokens are primitive types which can be threaded between side-effecting operations to order them. This CL changes infeed and outfeed to take a token as an operands and produce a token as one of its outputs. The most disruptive aspect of this change is that infeed now produces a two-element tuple containing the data value and a token. This means the shape of infed data no longer is the same as the shape of the infeed instruction, and a get-tuple-element operation must be called on the infeed instructions output to get its data.

Related changes/notes:

- The computation builder interface is unchanged. The infeed builder constructs an infeed instruction followed by a GTE instruction to extract the data value. Client and computation builder interface changes will be in follow up cls.

- Tokens can now be the root of the entry computation. Previously tokens could not be passed into or out of the entry computation. But now that outfeed produces a token, this constraint meant that outfeed could not be a root which is awkward. In the future we'd like to pass in tokens as well, perhaps as the only way of generating the initial token to thread through side-effecting ops.

- Infeed and outfeed still have a form which does not take a token to minimize the size of this CL. In the future this form will be removed. However, most HLO tests using infeed/outfeed are changed to accept a token in this cl.

PiperOrigin-RevId: 202041518