Separate Code Generation and Constant Collection during Compile

[gcatron]

Background:
With the new runtime design (#2045) we split the compilation step into two parts parts, code generation and collecting constants. Code generation is when the graph is converted to executable code that will run on the platform. Collecting Constants is the process of gathering weights and constants from the input module and copying them into a single chunk of memory for execution and copying to the device.

Impact:
This means we need to introduce a new method to the Backend interface. Namely, compileWithoutConstants. This does everything compile does except collect constants to one region of memory.