llama_model_loader: support multiple split/shard GGUFs (#6187)

llama_model_loader: support multiple split/shard GGUFs (#6187)

* split: support in llama_model_loader

* avoid copying the entire vector

Co-authored-by: slaren <redacted>
* split: move llama_tensor_offset to llama_model_loader

* llama_model_loader: PR feedbacks:
 - use only one gguf_context for metadata only
 - store all ggml_context in a vector as the files and mappings
 - store all weights in a vector along with the source tensor
 - rename ctx_gguf to meta
 - rename ctx_meta to contexts

* avoid copying the entire vector

* Simplify this by making these optional, switch some layer creation tensor optional

Co-authored-by: Georgi Gerganov <redacted>
* Handle optional tensors

Co-authored-by: Georgi Gerganov <redacted>
* llama_model_loader: fail if backend cannot allocate buffer

* fix mmap buffer management

* llama_model_loader: map file to backend buffer if the allocation succeeds only

* llama_model_loader: only map tensors included in the context

* llama_model_loader: minor, use same variable name for consistency, fix spacing in types cast

* llama_model_loader: fail if any of backend buffer cannot be allocated

* spacing

Co-authored-by: slaren <redacted>
* fix loop over pointer

Co-authored-by: slaren <redacted>
* llama_model_loader: if n_tensors declared not equals to loaded tensors in split, throw an exception instead of asserting

* llama_model_loader: ensure mappings vector has the expected size

* llama_model_loader:  use at instead of operator[] if this should never add to the map.

* llama_model_loader: immediately add the backend buffer to the model buffers in order to free them if an error occurs in the next allocation. Reserve the expected size.

* llama_model_loader: be sure the model mappings has enough capacity before allocating backend buffer

* llama_model_loader: fix map -> unordered map

* llama_split_prefix: use a clearer version, not pass split path len but dest max len.

Co-authored-by: Xuan Son Nguyen <redacted>
* llama : minor

ggml-ci

* llama : introduce some typedef helpers

* docs: add model shard in hot topic

* llama_model_loader: put mapping in a unique_ptr from the moment it is allocated

Co-authored-by: slaren <redacted>
* fix llama_split_prefix

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Co-authored-by: slaren <redacted>
Co-authored-by: Georgi Gerganov <redacted>
Co-authored-by: Xuan Son Nguyen <redacted>