[CI] Add functorch testing shard (#81283)
- Adds `functorch` pinned hash to the repo
- Adds `functorch` shard to `linux-bionic-py3.7-clang9`
- Adds `checkout_install_functorch` to `.jenkins/pytorch/common_utils.sh`
- Invokes this function during the test shard of functorch
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81283
Approved by: https://github.com/zou3519
