Fix qualx app metrics

[leewyang]

From @eordentlich, this PR fixes per-app aggregations, specifically for QX (raw xgboost) metrics on datasets where the sqlIDs may be misaligned.

Changes

1. defer GPU appDuration calculations to per-app aggregations post prediction vs. carrying per-sql pre-prediction.
2. add scaleFactor to the per-app aggregations.
3. add description to the appName when overridding descriptions to allow CPU/GPU joining.
4. fix loading model by name.

Test

Following CMDs have been tested:

Internal Usage:

python qualx_main.py predict
python qualx_main.py evaluate

[amahussein]

Thanks @leewyang