llama : refactor llama_context, llama_kv_cache, llm_build_context (v2)

[ggerganov]

alt #11213

Overview

The implementation in #11213 became too complicated, trying to make a lot of changes at once. This is an alternative implementation, which does not involve the abstraction of the llama_context. The PR introduces some new abstractions, improves the graph build handling and is an initial step for the next changes listed in section "Next" below.

• Rework the old llm_build_context into new llm_graph_context implemented in llama-graph.h/.cpp
• Introduce llm_graph_input_... classes for handling graph inputs in a safer and cleaner way
• Introduce llm_graph_result for extracting important tensors such as embeddings and logits, instead of searching for them by tensor name
• Introduce llm_memory_i concept that will abstract different cache/memory mechanisms. For now we have only llama_kv_cache as a type of memory
• Rework session saving/loading using new llama_io_write_i and llama_io_read_i interfaces
• Remove "worst case" concept from the graph building logic

API changes

The current changes are only necessary to make the API more consistent in following the naming convention. To migrate, simply replace the old API calls with the new ones.

• Deprecate llama_kv_cache_... API
• Add llama_kv_self_... API

Next

• Introduce new model arch interface and have the different models implement it
• Add new class llama_kv_cache_recurrent and remove all recurrent logic from the existing class llama_kv_cache_unified. Simplify llama_kv_cache_unified.

[ggerganov]

Planning to merge this tomorrow unless there are any suggestions for improvements.