Can we create a task which runs before the flow ends, when all upstream task are successful. (without mentioning all upstream tasks name)

I think you really need to either use

set_upstream

or

set_downstream

with the other tasks because if there is no dependency set, then it can be run in parallel. What does this task do? Your other option is to use a State Handler and run some code before the Flow ends but this is not a task

Here is my flow, with all upstreams task defined. When this whole flow is completed and marked successfull i want to shutdown my machine.

Not really. You can use a state handler for that. Ot you can use a resource manager and still will just down when all of those complete