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

    [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: alibaba#2040
    [Docs:Update] Update inference api usage.
    [Pymnn:Bugfix] Close hiai load to fix resource leak.
    [MNN:Update] Down gradle version for demo compile

Merge pull request alibaba#2037 from alibaba/feature/sync

[MNN:Sync] Sync Internal 2.1.0

Merge pull request alibaba#2027 from wangzhaode/feature/mnn_sync

Sync Internal 2.0.5

Merge pull request alibaba#2017 from alibaba/feature/sync

Sync Internal 2.0.4

Merge pull request alibaba#2000 from alibaba/feature/sync

[Sync] Sync internal 2.0.3

Merge pull request alibaba#1997 from alibaba/feature/sync

[MNN:Sync] Sync internal Gitlab to 2.0.2

Merge pull request alibaba#1986 from alibaba/feature/sync

[MNN:Sync] Sync 2.0.1

fix windows compile on avx512

Merge pull request alibaba#1952 from alibaba/feature/sync

[MNN:Sync] Sync internal gitlab

Fix bug for python3 can't run convertOnnxTest