Need to support the requirement of not creating and destroying variables in every run

[Xreki]

In the current implementation of Executor.Run, all the non-persistable variables will be created and destroyed in every Run. However, users may want to hold the non-persistable variables for two reasons:

• case 1, there is no memory problem and users want to save for runtime of frequent creating and destroying of variables.
• case 2, sometimes users may want to get the tensor of all operators for debug purpose.

As a result, we need to add an argument in Executor.Run to support these kinds of user requirements.