[core][1/N] Make executor to be a long-running Python thread

[kevin85421]

Why are these changes needed?

This is the same as #50644

Currently, tasks are sometimes executed on the main thread and sometimes on other threads which is created by C++. However, C++ threads are only considered Python threads when they execute Python functions. Hence, when a task finishes, the thread-local state will be garbage-collected. See #46336 (comment) for more details.

This PR uses the CPython API to treat C++ threads as Python threads, even if they do not execute Python functions.
https://docs.python.org/3/c-api/init.html#non-python-created-threads

This PR handles synchronous tasks. After this PR is merged, the follow-up tasks are:

• Always create a default executor.
• Move the constructor to the default executor.
• Support asynchronous tasks.

Related issue number

Part of #46336

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[kevin85421]

Are we certain that the gstate object does not need to be reference counted? Just want to make sure there is no concern of crashing during the shutdown sequence due to a memory corruption issue.

#50644 (review)

1. https://github.com/python/cpython/blob/75f38af7810af1c3ca567d6224a975f85aef970f/Include/pystate.h#L76-L78 PyGILState_STATE is merely an enum. It is not a PyObject, so it is not managed by reference counting.

2. I ran this example 100 times and all of them pass without a memory corruption issue.

[kevin85421]

I messed up the commits in the old PR, so I created a new one. Additionally, this comment contains details of the investigation I conducted on PyGILState_STATE.

[edoakes]

LGTM, thanks!