common : refactor common_sampler + grammar logic changes

[ggerganov]

from #17004

Extracting some refactoring portions from #17004 to make the review easier:

• Simplify and make safer the management of llama objects (samplers, contexts, model)
• The common_init_result now also owns the sampler chains constructed during common_init_from_params()
• The sampler chains of common_init_result are constructed before the model and the context - we will need this for sampling : add support for backend sampling #17004 in order to optionally pass the samplers during the construction of the context

---

ref #17750 (comment)

Another change related to the grammar logic (the explanation is in the referenced comment):

• No longer maintain a separate sampler chain for the grammar
• Merge the grammar into the main common_sampler chain
• The grammar is now always applied first to the raw logits, before the rest of the samplers

The main reason for this change is to make the integration of #17004 compatible with grammar usage and to simplify the logic for handling the grammar when it is present. The main concern is that this will likely hurt the performance when grammar sampling is involved, since we no longer do the "rejection sampling" trick. I think it's better to put effort to optimize the performance of the grammar in general so we don't need to do the trick at all.