ggml : add batched tensor copy API (#30881)
* ggml : add generic batched tensor copy API
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* llama : use batched tensor copy API in context io
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* ggml : add batched tensor copy API
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* llama : batch device io tensor copies
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* ggml : add CUDA support for batched tensor APIs
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* ggml : remove async tensor_n APIs and add null buffer hooks
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* ggml : fix batched tensor 2D detection
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* ggml : group batched tensor operations by buffer type
Assisted-by: pi:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* cont : minor indentantion
* cont : const correctness
* tests : add mock buffer type tests for batched tensor API
Restructure test-backend-tensor to run a unified test set on each backend buffer type and on an ad-hoc mock buffer type. The mock set exercises the dispatch logic in ggml_backend_tensor_{set,get,copy}_n: hook invocation, per-tensor fallback, cpy_tensor_n all-or-nothing, and buft grouping.
Assisted-by: dsh:llama.cpp/ggml-org/DeepSeek-V4-Flash-Vision-Exp-GGUF (Mac Studio, M2 Ultra, 192GB)
* cont : add TODO
