Add torchvision maintainers guide (#7109)

Co-authored-by: Philip Meier <github.pmeier@posteo.de>

maintainer_guide.md

+## Torchvision maintainers guide
+
+This document aims at documenting user-facing policies / principles used when
+developing and maintaining torchvision. Other maintainer info (e.g. release
+process) can be found in the meta-internal wiki.
+
+### What is public and what is private?
+
+For the Python API, torchvision largely follows the [PyTorch
+policy](https://github.com/pytorch/pytorch/wiki/Public-API-definition-and-documentation)
+which is consistent with other major packages
+([numpy](https://numpy.org/neps/nep-0023-backwards-compatibility.html),
+[scikit-learn](https://scikit-learn.org/dev/glossary.html#term-API) etc.).
+We recognize that his policy is somewhat imperfect for some edge cases, and that
+it's difficult to come up with an accurate technical definition. In broad terms,
+which are usually well understood by users, the policy is that:
+
+- modules that can be accessed without leading underscore are public
+- objects in a public file that don't have a leading underscore are public
+- class attributes are public iff they have no leading underscore
+- the rest of the modules / objects / class attributes are considered private
+
+The public API has backward-compatible (BC) guarantees defined in our
+deprecation policy (see below). The private API has not BC guarantees.
+
+For C++, code is private. For Meta employees: if a C++ change breaks fbcode, fix
+fbcode or revert the change. We should be careful about models running in
+production and relying on torchvision ops.
+
+The `test` folder is not importable and is **private.** Even meta-internal
+projects should *not* rely on it (it has happened in the past and is now
+programmatically impossible).
+
+The training references do not have BC guarantees. Breaking changes are
+possible, but we should make sure that the tutorials are still running properly,
+and that their intended narrative is preserved (by e.g. checking outputs,
+etc.).
+
+The rest of the folders (build, android, ios, etc.) are private and have no BC
+guarantees.
+
+### Deprecation policy.
+
+Because they're disruptive, **deprecations should only be used sparingly**.
+
+We largely follow the [PyTorch
+policy](https://github.com/pytorch/pytorch/wiki/PyTorch's-Python-Frontend-Backward-and-Forward-Compatibility-Policy):
+breaking changes require a deprecation period of at least 2 versions.
+
+Deprecations should clearly indicate their deadline in the docs and warning
+messages. Avoid not committing to a deadline, or keeping deprecated APIs for too
+long: it gives no incentive for users to update their code, sends conflicting
+messages ("why was this API removed while this other one is still around?"), and
+accumulates debt in the project.
+
+### Should this attribute be public? Should this function be private?
+
+When designing an API it’s not always obvious what should be exposed as public,
+and what should be kept as a private implementation detail. The following
+guidelines can be useful:
+
+* Functional consistency throughout the library is a top priority, for users and
+  developers’ sake. In doubt and unless it’s clearly wrong, expose what other
+  similar classes expose.
+* Think really hard about the users and their use-cases, and try to expose what
+  they would need to address those use-cases. Aggressively keep everything else
+  private. Remember that the “private -> public” direction is way smoother than
+  the “public -> private” one: in doubt, keep it private.
+* When thinking about use-cases, the general API motto applies: make what’s
+  simple and common easy, and make what’s complex possible (80% / 20% rule).
+  There might be a ~1% left that’s not addressed: that’s OK. Also, **make what’s
+  wrong very hard**, if not impossible.
+
+As a good practice, always create new files and even classes with a leading
+underscore in their name. This way, everything is private by default and the
+only public surface is explicitly present in an `__init__.py` file.