Make getRank abort for unranked type

    This better matches the other methods in ShapedType which only make sense for ranked types. There's now an explicit hasRank for checking the rank. Actual call sites rarely used the "-1" sentinel to combine checking for rankedness and checking that rank is a certain value. And in most cases they should actually be checking for rankedness at a higher level using type predicates. Using an explicit method is clearer than a sentinel anyway.

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PiperOrigin-RevId: 250720853