Update to gotch 0.9.0

Would be great to update transformer to work with https://github.com/sugarme/gotch/tree/v0.9.0

For example in `/transformer/util/activation.go`, the MustGelu method signature changed to accept a string and a bool:
```
func (g GeluActivation) Fwd(x *ts.Tensor) (retVal *ts.Tensor) {
	return x.MustGelu("???", false)
                           ^^^
}
```