[MNN:Sync] Sync Internal 2.1.1 contain below changes.

[wangzhaode]

[Pymnn:Bugfix] Fix usage and small bug in pymnn.
[Docs:Update] Update docs/cpp markdown
[Docs:Update] Add docs check.
[MNN:Update] Update VecHalf.hpp
[MNN:Bugfix] Fix compile errors caused by "#define MNN_THREAD_LOCK_CPU"
[Geometry:Bugfix] Fix bug for resize of broadcastto: https://github.com/alibaba/MNN/issues/2040
[Docs:Update] Update inference api usage.
[Pymnn:Bugfix] Close hiai load to fix resource leak.
[MNN:Update] Down gradle version for demo compile

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