Use correct workspace alloc call in MIOpen conv operator (#15712)
Summary:
This PR contains changes for:
1. Using memory alloc from HIPContext while allocating workspace for MIOpen conv and transpose_conv operators rather than direct HIP mem alloc
2. Minor cleanup and removing an unnecessary sync call from MIOpen conv op
Differential Revision: D13598894
Pulled By: bddppq
fbshipit-source-id: 44886161abdf91cd29c7c93b3e23620e1b09c7c9
