sampling : avoid expensive softmax during greedy sampling (#9605)

* sampling : avoid expensive softmax during greedy sampling

ggml-ci

* speculative : fix default RNG seed + set sparams.n_probs

* Update tests/test-sampling.cpp

Co-authored-by: slaren <slarengh@gmail.com>

* sampling : add clarifying comment [no ci]

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Co-authored-by: slaren <slarengh@gmail.com>