ml: Remove Output from Context interface

Model implementations should use Input for all of their tensors
supplied to the model. This includes tensors that relate to the
outputs, which is confusing since there is also an Output funciton.

Since Output is only used internally in GGML and not used by any
model implementations, we can remove it from the interface to
reduce confusion.

ml/backend.go

@@ -110,12 +110,10 @@ type Context interface {
 	MaxGraphNodes() int
 	Close()
 
-	// Input returns a context appropriate for creating input tensors
+	// Input returns a context appropriate for creating tensors that are
+	// inputs to the model (which includes things like output locations)
 	Input() Context
 
-	// Output returns a context appropriate for creating output tensors
-	Output() Context
-
 	// Layer returns a context appropriate for creating intermediate tensors
 	Layer(int) Context
 }