Change instance type in CI to use G4 instance

[sandeep-krishnamurthy]

This still require experimentation on time v/s cost v/s completeness. One of the proposal is to use AWS G4 instances instead of current P2/P3 instances for GPU tests.

[sandeep-krishnamurthy]

@ChaiBapchya @josephevans

[ChaiBapchya]

This has been completed. Unix-gpu builds use G4 since May 11
#18186