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* Conditionally include Python in the C++ builds.

Added an option `USE_PYTHON` that allows users to decide to link to Python3 or not.

* Also conditionally include this defintion.

* Define `USE_PYTHON` for the Windows builds.

* Remove Python3 reference in CMake test as it's no longer required.

* Accidentally removed the closing > from the decl.

* Also add `USE_PYTHON` when building the separate image library on Windows.

* Also conditionally include this depening on USE_PYTHON.

* Go back to require Python for this example.

* Find Python in the hello world example.

* Add a paragraph documenting the `USE_PYTHON` option.
Co-authored-by: Prabhat Roy <prabhatroy@fb.com>

* add api usage log for io

* cover VideoReader

* cover c++ APIs

* add api usage log for io

* cover VideoReader

* cover c++ APIs

* add _cpp suffix to c++ APIs

* use new API and change cpp format

* remove _cpp suffix

* adopt new API

* lint

* Moving registration of video methods in Video.cpp and removing unnecessary includes.

* Rename files according to cpp styles.

* Adding namespaces and moving private methods to anonymous namespaces.

* Syncing method names.

* Fixing minor issues.

Summary:
* Reduce unnecessary header inclusions in models and io.

* Move autocast to separate folder and hide autograd implementation in an anonymous namespace.

* Moving files in subfolders.

Reviewed By: fmassa

Differential Revision: D25461523

fbshipit-source-id: 756eeb6848aacaa474de4825ed4c1045d17e2cea