TP: fix entirely zero-sized slices per device (#23525)

ggml/include/ggml-alloc.h

@@ -76,6 +76,7 @@ GGML_API size_t ggml_gallocr_get_buffer_size(ggml_gallocr_t galloc, int buffer_i
 // Utils
 // Create a buffer and allocate all the tensors in a ggml_context
 // ggml_backend_alloc_ctx_tensors_from_buft_size returns the size of the buffer that would be allocated by ggml_backend_alloc_ctx_tensors_from_buft
+// ggml_backend_alloc_ctx_tensors_from_buft returns NULL on failure or if all tensors in ctx are already allocated or zero-sized
 GGML_API size_t                       ggml_backend_alloc_ctx_tensors_from_buft_size(struct ggml_context * ctx, ggml_backend_buffer_type_t buft);
 GGML_API struct ggml_backend_buffer * ggml_backend_alloc_ctx_tensors_from_buft(struct ggml_context * ctx, ggml_backend_buffer_type_t buft);
 GGML_API struct ggml_backend_buffer * ggml_backend_alloc_ctx_tensors(struct ggml_context * ctx, ggml_backend_t backend);