Improve the design of the `compiler` library

[wmdi]

This issue is to track the remaining TODOs to improve the design of compiler.

• Refactor the representation of the results of different levels of search, e.g., Strategy. It seems unreasonable to put runtime as part of Strategy since it is determined by pcg and machine_mapping. Also, whether to use visitable for Strategy should be considered.
• Correctly implement estimate_cost. The current version does not take parallelization into account.
• Separate the problem logic (e.g., dp states) from the search logic (e.g., MCMC and dynamic programming), and separate the exploration logic from the evaluation logic. Need to consider how to design the interfaces. It might be a large PR for this.
• Improve readability of graph testcases.

Eventually it would be good to add infrastructure to include dot descriptions of these graphs in these testcases for readability accompanied with checks to make sure they don't get out of date. It might even be worth writing a quick parser and DSL for specifying them as otherwise it's hard to tell what these testcases are actually doing (at least for people who didn't write the testcases themselves 🙂 )

• Improve the hash function for ParallelComputationGraph.

[lockshaw]

@wmdi When you divide up this issue can you make sure to make individual issues for (1) separating up the DP, and (2) separating up the MCMC search? I'm guessing it'll be easier to handle them individually rather than solving both at once

[lockshaw]

@wmdi Can you also make sure the rapidcheck issues you ran into get a corresponding issue created?

[lockshaw]

@wmdi Can you make sure that a more detailed issue gets created for https://reviewable.io/reviews/flexflow/FlexFlow/1229#-NpXEwmgBs4Q6-g0dSIh? This issue doesn't really mention it currently

[lockshaw]

@wmdi Can you make sure https://reviewable.io/reviews/flexflow/FlexFlow/1229#-NpXFKHVCMVgHr2GOPAK gets linked to in whatever issue(s) get created for graph hashing?

[lockshaw]

@wmdi Can you make sure that https://reviewable.io/reviews/flexflow/FlexFlow/1229#-NpXIiYi1DIya7jad3kQ gets linked in the issue that you create for handling variants in the graph library?