fit : account for skipped NextN layers when setting n_gpu_layers (#31569)
* fit : account for skipped NextN layers when setting n_gpu_layers
Without MTP, fit counts n_layer + 1 layers while the model loader counts
n_layer_all + 1 for n_gpu_layers, including the NextN layers which hold no
weights. A fit result that offloads all layers then leaves layer 0 on the CPU
("offloaded 65/66 layers" for Qwen3.8-27B on two GPUs).
Add the skipped NextN layers to n_gpu_layers and to the tensor split of the
device that holds the output layer so the loader assigns the layers that
fit measured.
Fixes #31531
* fit : pass hp_ngl_skip by reference like the other hparams
* fit : add hp_ngl_skip to common_get_device_memory_data