[bazel] make it possible to build the whole world, update CI (#78870)
Fixes https://github.com/pytorch/pytorch/issues/77509
This PR supersedes https://github.com/pytorch/pytorch/pull/77510.
It allows both `bazel query //...` and `bazel build --config=gpu //...` to work.
Concretely the changes are:
1. Add "GenerateAten" mnemonic -- this is a convenience thing, so anybody who uses [Remote Execution](https://bazel.build/docs/remote-execution) can add a
```
build:rbe --strategy=GenerateAten=sandboxed,local
```
line to the `~/.bazelrc` and build this action locally (it doesn't have hermetic dependencies at the moment).
2. Replaced few `http_archive` repos by the proper existing submodules to avoid code drift.
3. Updated `pybind11_bazel` and added `python_version="3"` to `python_configure`. This prevents hard-to-debug error that are caused by an attempt to build with python2 on the systems where it's a default python (Ubuntu 18.04 for example).
4. Added `unused_` repos, they purpose is to hide the unwanted submodules of submodules that often have bazel targets in them.
5. Updated CI to build //... -- this is a great step forward to prevent regressions in targets not only in the top-level BUILD.bazel file, but in other folders too.
6. Switch default bazel build to use gpu support.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78870
Approved by: https://github.com/ezyang
