Call ts.Drop() ts.MustDrop() multiple times

[sugarme]

Make ts.Drop() can be called multiple times.

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
}