backend: Don't return an error on Close

It is not common to return errors with close/free operations - most
people won't check it and even if they did there's probably not much
that can do. It's better to not give implementations false expectations.

ml/backend.go

@@ -50,7 +50,7 @@ type Context interface {
 
 	Forward(Tensor)
 	Compute(Tensor) Tensor
-	Close() error
+	Close()
 }
 
 type Tensor interface {

ml/backend/ggml/ggml.go

@@ -306,10 +306,9 @@ func (c Context) FromIntSlice(s []int32, shape ...int) (ml.Tensor, error) {
 	return fromSlice(c, s, shape, C.GGML_TYPE_I32)
 }
 
-func (c *Context) Close() error {
+func (c *Context) Close() {
 	C.ggml_backend_sched_free(c.sched)
 	C.ggml_free(c.ctx)
-	return nil
 }
 
 type Tensor struct {