Task Fusion, Constant Conversion Optimization, and 27pt stencil benchmark

[shivsundram]

PR for implementing Legate Task Fusion.
This is the cuNumeric companion PR to the Core's Task Fusion PR nv-legate/legate#113
This PR contains 3 primary changes

1. Constant optimization for binary ops, implemented in cunumeric/array.py This removes the need to issue expensive "convert" ops for scalar constants embedded in the code
2. Changes to the black scholes benchmark so NaN checking is only done once all benchmarking runs have finished
3. New 27 pt stencil benchmark
4. Implementation of Task Fusion: Currently this exists as a cunumeric task, but the intention is too eventually move it to the core

[magnatelee]

Thanks for the PR. I'll probably split this into a couple of independent PRs instead of merging it as-is, and want to move all fusion related code to the core.

[shivsundram]

above code is a scalar constant optimization, which avoids dispatching CONVERT operations (for a scalar constant), as the constant's value is embedded in the code and thus already known

[marcinz]

@magnatelee @shivsundram What is the status of this PR?

[magnatelee]

@marcinz same here. part of this PR should really be in the core, so I'll do the porting in the near future.

[shivsundram]

@marcinz @magnatelee Yeah this is/was a working PR (with some nice speedup results here if interested), but Wonchan will be porting/merging this functionality into the core. This PR is pretty stale right now
That said, the constant optimizations in this PR - that prevents dispatching expensive 'convert' Legate ops for converting the format (eg fp32->fp64) of already existing scalars - may be worth merging in earlier (if not already done). It's a smaller change and the speedups from it were quite tangible