[AutoScheduler] Improve the GPU tutorial by deleting measure_ctx earlier

[merrymercy]

To avoid the confusion in #6570, we call del measure_ctx earlier in the tutorial, just after we finish the search.

measure_ctx overloads the __del__. Ideally, we do not need to call del measure_ctx explicitly.
However, I found __del__ only works for local variables.
If we use measure_ctx as a local variable inside a function, it can be garbage-collected correctly.
But in the tutorial, measure_ctx is a global variable. It thus cannot be garbage-collected correctly.

There are two solutions to this problem.
S1. call del measure_ctx explicitly
S2. use

with measure_ctx:
  tune_options = ...
  s, args = auto_scheduler.auto_schedule(...)

I think S1 is better. In S1, del measure_ctx is only required if measure_ctx is a global variable, which is a rare case (but unfortunately, this happens in the tutorial)

[merrymercy]

cc @jcf94 @luchangli03

[comaniac]

LGTM