wait() failing for the default stream 0

[lakhinderwalia]

This PR fixes these two issues when using the default stream on the GPU: i.e. via MIGRAPHX_ENABLE_NULL_STREAM=1:

1. Trying to verify on a model, (resnet50v2 here) gives this exception: context.hpp:365: get_elapsed_ms: Failed hipEventElapsedTime: device not ready

2. Trying to run perf on a model with the default stream gives more optimistic results, as the gpu-sync-up is essentially bypassed, incorrectly.

[amd-jmacaran]

/AzurePipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.40%. Comparing base (b235f15) to head (57e384a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3873   +/-   ##
========================================
  Coverage    92.40%   92.40%           
========================================
  Files          520      520           
  Lines        22485    22485           
========================================
  Hits         20778    20778           
  Misses        1707     1707           

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[CharlieL7]

LGTM

[pfultz2]

Since this relies on undocumented behavior, a unit test needs to be added.