Introduce cuFFT plan cache; switch to auto-managed memory.

[maleadt]

This PR introduces a cache for CUFFT handles, much like how cupy recently did.

Before:

julia> @benchmark fft($A)
BenchmarkTools.Trial: 1166 samples with 1 evaluation.
 Range (min … max):  2.479 ms … 281.481 ms  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     3.999 ms               ┊ GC (median):    0.00%
 Time  (mean ± σ):   4.274 ms ±   8.145 ms  ┊ GC (mean ± σ):  0.00% ± 0.00%

                    ▃ ▁  ▁ ▂▄█▇▃▃▅▂▄▃▃ ▁ ▁▂▃▂▅ ▁  ▁
  ▃▁▁▁▂▁▂▃▁▃▄▃▆▇▇████▇██▆███████████████▇███████▇██▇▇▇▄▅▄▃▃▃▃ ▅
  2.48 ms         Histogram: frequency by time        5.26 ms <

 Memory estimate: 2.81 KiB, allocs estimate: 64.

After:

julia> @benchmark fft($A)
BenchmarkTools.Trial: 10000 samples with 1 evaluation.
 Range (min … max):   84.129 μs … 805.442 μs  ┊ GC (min … max):  0.00% … 91.37%
 Time  (median):     275.457 μs               ┊ GC (median):    87.83%
 Time  (mean ± σ):   276.244 μs ±   9.010 μs  ┊ GC (mean ± σ):  87.78% ±  0.98%

                                                           █▂
  ▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▂██▄▂ ▂
  84.1 μs          Histogram: frequency by time          286 μs <

Reference:

❯ python wip.py
fft_func            :    CPU:   94.263 us   +/-16.328 (min:   68.750 / max:  421.615) us     GPU-0:  110.159 us   +/-31.509 (min:   42.400 / max: 2295.744) us

Closes #1682.

[codecov]

Codecov Report

Base: 59.69% // Head: 59.71% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (d90a32c) compared to base (c35bc08).
Patch coverage: 93.52% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1734      +/-   ##
==========================================
+ Coverage   59.69%   59.71%   +0.02%     
==========================================
  Files         150      150              
  Lines       12067    12077      +10     
==========================================
+ Hits         7203     7212       +9     
- Misses       4864     4865       +1     

Impacted Files	Coverage Δ
lib/utils/cache.jl	94.28% <90.90%> (-1.55%)	⬇️
lib/cufft/fft.jl	83.92% <93.54%> (-3.67%)	⬇️
lib/cufft/wrappers.jl	94.11% <93.81%> (-5.89%)	⬇️

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