- Cache the vector sizes in IExecutionFrame and NodeIndexInfo to avoid calls to size().
  - 2 instructions instead of 10
- Remove an unnecessary check in IExecutionFrame
  - add a check to the ctor so we guarantee it's unnecessary
- Reserve memory for the vectors in BroadcastIterator
  - saves reallocs if more than one value is added
    - but rare with the mlperf models for multiple values to be added so benefit is limited.
  - slight tweak to the Broadcaster ctor code to make it more readable