eval-callback: use ggml_op_desc to pretty print unary operator name

[phymbert]

Context

Unary operator name was not printed.

Suggested by @slaren here: #6576 (comment)

Before:

ggml_debug:           ffn_moe_silu-0 = (f32)      UNARY(ffn_moe_gate-0{10752, 3, 1, 1}, }) = {10752, 3, 1, 1}
                                     [
                                      [
                                       [  0.0065,   4.0065,   8.0065, ...],
                                       [43008.0078, 43012.0078, 43016.0078, ...],
                                       [86016.0078, 86020.0078, 86024.0078, ...],
                                      ],
                                     ]
                                     sum = 387108.062500

After:

ggml_debug:           ffn_moe_silu-0 = (f32)       SILU(ffn_moe_gate-0{10752, 3, 1, 1}, }) = {10752, 3, 1, 1}
                                     [
                                      [
                                       [  0.0065,   4.0065,   8.0065, ...],
                                       [43008.0078, 43012.0078, 43016.0078, ...],
                                       [86016.0078, 86020.0078, 86024.0078, ...],
                                      ],
                                     ]
                                     sum = 387108.062500