Improve Python API including enabled AsynchronousOut mode

[frankier]

Currently AsynchronousOut mode won't work with the Python API since it is not possible to get ahold of a Datum created inside OpenPose from the Python side. The reason for this is at the moment, lists are copied when going over the Python => C++ boundary. See https://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html#making-opaque-types . To get the semantics of passing a mutable reference, we can wrap an STL type.

Unfortunately this is not quite sufficient. We need to pass a shared_pointer<vector<shared_pointer>> to openpose::Wrapper. The outer shared pointer is eventually pointed somewhere else in queue.hpp:

openpose/include/openpose/thread/queue.hpp

Line 73 in 3915b92

tDatums = {std::move(this->mTQueue.front())};

So we just swap that vector with ours, which should avoid copying the datum I think. I'm a bit confused about why there are so many levels of nesting though. A more idiomatic approach would be to just return a value in Python, but since the Python/C++ APIs are currently very similar, I thought I'd keep it this way and just adjust it to work in this case also.

This PR also contains the following related changes:

• Update Python tutorial for adjusted API and port C++ AsynchronousOut example to Python
• Add ThreadManagerMode enum to Python interface

And a couple of unrelated bug fixes to the Python API:

• Add type_caster<op::Array<long long>> so datum.poseIds can be accessed from Python
• Cast empty Arrays on Datum to None in Python rather than numpy scalars of trash data.

[frankier]

Updated to only set up producer when we need synchronous input

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[frankier]

Please review!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[frankier]

Please review!

[gineshidalgo99]

Sorry for the HUGE delay! I have finally reviewed it, there is a big question I have:

The new Python example you added (examples/tutorial_api_python/12_asynchronous_custom_output.py), how is it an asynchronous example? Where in that example you set it to be asynchronous?

[frankier]

You're right -- that seems to have been an oversight. Fixed.

[frankier]

Is there anything I can do to make this easier to review and merge?

[gineshidalgo99]

Pushed, this is an amazing PR, thanks a lot! (It just took me forever to review it given its complexity, sorry about that, but amazing PR!)