Add C++ coverage for Ubuntu cpu tests (#46656)
Summary:
In order to enable C++ code coverage for tests, we need to build pytorch with the correct coverage flags. This PR should introduce a build that allows coverage tests to stem from a specific coverage build.
This PR does the following:
1. Adds a new build to `*-coverage_*` build with the correct `--coverage` flag for C++ coverage
2. Calls `lcov` at the end of testing to capture C++ coverage results
3. Pushes C++ results along with Python results
4. Shards the coverage test to not take ~4hrs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46656
Reviewed By: walterddr
Differential Revision: D24636213
Pulled By: janeyx99
fbshipit-source-id: 362a1a2a20c069ba0a7931669194dac53ac81133
