What is the expected enhancement?

Subtasks to clean up the compilation flow, make it more modular, parallelize it, etc.

• Separate Circuits from DAGs from Qobj, only work with dags in the transpile (PR Compile split up for transpile passes #705).
• Make sure dag has all the fields that circuits have so circuits don't need to be passed around (PR Compile split up for transpile passes #705).
• Improve the pass-based transpiler to support advanced functionality (Issue Improve the pass-based transpiler to support advanced functionality #825, PR Transpiler to support advanced functionality #824 )
• Remove the other output formats of transpile and only support dag (Move visualizers out of JSON #1129)
• make sure transpile only uses pases. (Default transpiler pipeline #1161)
• Think about whether we want compilation of a batch where coupling_map, basis_gates, or initial_layout are different for each circuit. -- decided only one for batch.
• Swap mapper pass (PR [WIP] Swap mapper pass #757)
• Unroll pass
• Rotation merge pass
• A* mapper pass (Issue Add A*-based mapper #750)
• Tree search swap mapper pass (pr Add tree search swap mapper #1140 )
• Choose Layout pass (DenseLayout and TrivialLayout pass #1499)
• KAK decomposition pass (kak decomposition is needed #1580 )
• There are two seeds that do different things (simulator seed, optimizer seed and should not be the same)
• Remove hpc parameters from transpile() (Issue Usage of hpc parameters clarification #769)
• Use a single standard for intitial_layout all the way down.
• Make using_qiskit_level_2 and 3 work correctly by allowing compile once/execute twice on different backends and to show how to customize circuits with transpile