TST: Time the execution of operators during adjoint tests

[carterbox]

Purpose

@nikitinvv wants to know how long it takes for each of the operators to execute.

Approach

I have added walltime markers before and after execution of operators in the adjoint tests. I'm not sure that adding CUDA synchronization points was necessary, so I did not include them. The problem sizes for each operator can be adjusted by editing the setUp() function of each test. Not sure if it's necessary to allow changing the problem size from the command line.

Pre-Merge Checklists

Submitter

• [ ] Write a helpfully descriptive pull request title.
• [ ] Organize changes into logically grouped commits with descriptive commit messages.
• [ ] Document all new functions.
• [ ] Write tests for new functions or explain why they are not needed.
• [ ] Build the documentation successfully
• [ ] Use yapf to format python code.

Reviewer

• [ ] Actually read all of the code.
• [ ] Run the new code yourself.
• [ ] Write a summary of the changes as you understand them.
• [ ] Thank the submitter.

[carterbox]

I think the actual best way to time the operators is the use nvprof. I'm not sure that this approach is valid.