cast: return self for same dtype (#6306)

* cast: return self for same dtype

* cast: format
Co-authored-by: Noneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

oneflow/core/functional/impl/math_functor.cpp

@@ -346,6 +346,8 @@ class CastFunctor {
   CastFunctor() { op_ = CHECK_JUST(one::OpBuilder("cast").Input("in").Output("out").Build()); }
   Maybe<Tensor> operator()(const std::shared_ptr<one::Tensor>& x,
                            const Symbol<DType>& dtype) const {
+    if (x->dtype() == dtype) { return x; }
+
     MutableAttrMap attrs;
     JUST(attrs.SetAttr<DataType>("dtype", dtype->data_type()));
     return OpInterpUtil::Dispatch<Tensor>(*op_, {x}, attrs);