ml/backend/ggml: consolidate system info logging

- output backend system info when initializing the backend. this ensures
  this information is always present without needing to be called
  explicitly
- convert to structured logging
- enumerate devices rather than backends since devices are ordered
- track device indices grouped by device name

ml/backend.go

@@ -24,7 +24,6 @@ type Backend interface {
 	Config() Config
 	Get(name string) Tensor
 	NewContext() Context
-	SystemInfo() string
 }
 
 // BackendCacheConfig should be implemented by backends that need special output