Add DynamicApproxTopKOp to StableHLO
Add DynamicApproxTopKOp to StableHLO

This CL adds a DynamicApproxTopKOp to StableHLO. This op is a dynamic version of the ApproxTopKOp. The op is represented via a custom call to `stablehlo.dynamic_approx_top_k`. The custom call has the regular operand of ApproxTopKOp plus an additional `k` operand that determines the shape of the output.

The semantics of DynamicApproxTopKOp are inherited from semantics of ApproxTopKOp.
