Implements get_free_variable set functions

This MR implements various functions to get the set of the free variable of models, benchmarks, and packed_measurement. They are required for the benchmark dependency analysis.

Note that there are 2 kinds of the functions:

"Unapplied" or "raw" versions : Based on the free variables got from the model without applying any workload. This only covers those who are used inside the cost function definitions.

Applied versions : The free variables got from the model with applying a workload. This includes the free variables of the operations executed in the benchmark. Some of the free variables may be optimized and eliminated for some workloads, especially those for the intercept case benchmarks.