[train] add proper filtering to metrics

[matthewdeng]

1. Added GPU filtering:

   • Added GpuIndex and GpuDeviceName variables to dashboard
   • Updated GPU panels to use these variables
   • Variables hidden by default

2. Improved Train metrics:

   • Added TrainRunName and TrainRunId to all Train queries
   • Standardized quote usage
   • Removed {global_filters} in favor of explicit variables

3. Simplified Train Run view:

   • Reduced to only Train-specific metrics
   • Removed system resource panels