Avoid fusion precompilation after ASV 0.3.0 is available

(This is a reminder issue.)

Current code runs fuse precompilation for N^2 times (where N = number of `def test_*` benchmarks in `bench_fuse`) in one benchmark session (note that asv runs each benchmark in separate process). ASV 0.3.0 will introduce concept of warmup, so I'd like to use it.

https://github.com/cupy/cupy-benchmark/blob/6a8861922c1563d0bafdf6087943ffca711c7d73/benchmarks/cupy/bench_fusion.py#L39

Start with benchmarks/cupy/bench_fusion.py at the referenced line and review how ASV 0.3.0 warmup is configured. Confirm the benchmark session currently repeats fuse precompilation for each benchmark pair; done means precompilation is handled through warmup rather than repeated N² times.

Written by the indexing model from the issue text.