add `lcov` to oss for beautiful html report (#44568)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44568
By `lcov`, we can generate beautiful html. It's better than current file report and line report. Therefore in oss gcc, remove `export` code and `file/line level report` code, only use the html report.
But in clang, since such tool is not available, we will still use file-report and line-report generated by ourself.
Test Plan:
Test in docker ubuntu machine.
## Mesurement
1. After running `atest`, it takes about 15 mins to collect code coverage and genrate the report.
```
# gcc code coverage
python oss_coverage.py --run-only=atest
```
## Presentation
**The html result looks like:**
*Top Level:*
{F328330856}
*File Level:*
{F328336709}
Reviewed By: malfet
Differential Revision: D23550784
fbshipit-source-id: 1fff050e7f7d1cc8e86a6a200fd8db04b47f5f3e
