Set default compiler differently according to platform (#43890)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43890
1. auto-detect `CXX` default compiler type in oss, and `clang` as default compiler type in fbcode (because auto-detecting will say `gcc` is the default compiler on devserver).
2. change `compiler type` from str `"CLANG" "GCC"` to enum type
3. rename function `get_cov_type` to `detect_compiler_type`
4. auto-set the default pytorch folder for users in oss
Test Plan:
on devserver:
```
buck run :coverage //caffe2/c10:
```
on oss:
```
python oss_coverage.py --run-only=atest
```
Reviewed By: malfet
Differential Revision: D23420034
fbshipit-source-id: c0ea88188578bb1343a286f2090eb8a74cdf3982
