graph mode: add hardtanh op (#37469)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37469
Adds graph mode handling for hardtanh op.
Test Plan:
CI
Imported from OSS
Differential Revision: D21310362
fbshipit-source-id: 63e4ffca5cf2f345c2a66f84db2193a5e14b1028
