feat(tensor): ts.MustDrop() now can call multiple times

example/basic/main.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	"fmt"
+	// "fmt"
 
 	"github.com/sugarme/gotch"
 	ts "github.com/sugarme/gotch/tensor"
@@ -13,9 +13,4 @@ func main() {
 	tensor := ts.MustArange(ts.IntScalar(2*3*4), gotch.Int64, gotch.CPU).MustView([]int64{2, 3, 4}, true)
 
 	tensor.Print()
-
-	nilTs := ts.NewTensor()
-
-	fmt.Printf("nilTs val: %v", nilTs)
-
 }

tensor/tensor.go

@@ -873,11 +873,22 @@ func (ts Tensor) MustToString(lw int64) (retVal string) {
 
 // Drop drops (frees) the tensor
 func (ts Tensor) Drop() (err error) {
+
+	if !ts.MustDefined() {
+		return nil
+	}
+
 	lib.AtFree(ts.ctensor)
 	if err = TorchErr(); err != nil {
 		return err
 	}
 
+	// NOTE. assign to a new undefined tensor, then check `ts.MustDefined`
+	// before deleting at C land. Hence `Drop` method can be called
+	// multiple times without worrying about double C memory delete panic.
+	// Other pattern is `defer ts.MustDrop()` whenever a tensor is created.
+	ts = NewTensor()
+
 	return nil
 }