the streams created by cudax::stream should not synchronize with the null stream

[ericniebler]

Description

the null stream was a mistake. in cudax i don't think we should be allowing spooky action at a distance by creating more streams that synchronize with it.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[github-actions]

🟩 CI finished in 26m 07s: Pass: 100%/26 | Total: 2h 31m | Avg: 5m 48s | Max: 20m 38s | Hits: 69%/312

• 🟩 cudax: Pass: 100%/26 | Total: 2h 31m | Avg: 5m 48s | Max: 20m 38s | Hits: 69%/312

  🟩 cpu
    🟩 amd64              Pass: 100%/22  | Total:  2h 16m | Avg:  6m 13s | Max: 20m 38s | Hits:  69%/312   
    🟩 arm64              Pass: 100%/4   | Total: 14m 09s | Avg:  3m 32s | Max:  3m 41s
  🟩 ctk
    🟩 12.0               Pass: 100%/3   | Total: 19m 03s | Avg:  6m 21s | Max: 11m 29s | Hits:  69%/156   
    🟩 12.5               Pass: 100%/2   | Total: 12m 04s | Avg:  6m 02s | Max:  6m 18s
    🟩 12.6               Pass: 100%/21  | Total:  1h 59m | Avg:  5m 42s | Max: 20m 38s | Hits:  69%/156   
  🟩 cudacxx
    🟩 nvcc12.0           Pass: 100%/3   | Total: 19m 03s | Avg:  6m 21s | Max: 11m 29s | Hits:  69%/156   
    🟩 nvcc12.5           Pass: 100%/2   | Total: 12m 04s | Avg:  6m 02s | Max:  6m 18s
    🟩 nvcc12.6           Pass: 100%/21  | Total:  1h 59m | Avg:  5m 42s | Max: 20m 38s | Hits:  69%/156   
  🟩 cudacxx_family
    🟩 nvcc               Pass: 100%/26  | Total:  2h 31m | Avg:  5m 48s | Max: 20m 38s | Hits:  69%/312   
  🟩 cxx
    🟩 Clang9             Pass: 100%/1   | Total:  3m 44s | Avg:  3m 44s | Max:  3m 44s
    🟩 Clang10            Pass: 100%/1   | Total:  4m 45s | Avg:  4m 45s | Max:  4m 45s
    🟩 Clang11            Pass: 100%/1   | Total:  3m 53s | Avg:  3m 53s | Max:  3m 53s
    🟩 Clang12            Pass: 100%/1   | Total:  4m 09s | Avg:  4m 09s | Max:  4m 09s
    🟩 Clang13            Pass: 100%/1   | Total:  3m 56s | Avg:  3m 56s | Max:  3m 56s
    🟩 Clang14            Pass: 100%/1   | Total:  4m 21s | Avg:  4m 21s | Max:  4m 21s
    🟩 Clang15            Pass: 100%/1   | Total:  4m 05s | Avg:  4m 05s | Max:  4m 05s
    🟩 Clang16            Pass: 100%/1   | Total:  4m 16s | Avg:  4m 16s | Max:  4m 16s
    🟩 Clang17            Pass: 100%/1   | Total:  4m 31s | Avg:  4m 31s | Max:  4m 31s
    🟩 Clang18            Pass: 100%/4   | Total: 26m 13s | Avg:  6m 33s | Max: 15m 06s
    🟩 GCC9               Pass: 100%/1   | Total:  3m 50s | Avg:  3m 50s | Max:  3m 50s
    🟩 GCC10              Pass: 100%/1   | Total:  4m 18s | Avg:  4m 18s | Max:  4m 18s
    🟩 GCC11              Pass: 100%/1   | Total:  4m 20s | Avg:  4m 20s | Max:  4m 20s
    🟩 GCC12              Pass: 100%/2   | Total: 25m 09s | Avg: 12m 34s | Max: 20m 38s
    🟩 GCC13              Pass: 100%/4   | Total: 14m 28s | Avg:  3m 37s | Max:  3m 42s
    🟩 MSVC14.36          Pass: 100%/1   | Total: 11m 29s | Avg: 11m 29s | Max: 11m 29s | Hits:  69%/156   
    🟩 MSVC14.39          Pass: 100%/1   | Total: 11m 33s | Avg: 11m 33s | Max: 11m 33s | Hits:  69%/156   
    🟩 NVHPC24.7          Pass: 100%/2   | Total: 12m 04s | Avg:  6m 02s | Max:  6m 18s
  🟩 cxx_family
    🟩 Clang              Pass: 100%/13  | Total:  1h 03m | Avg:  4m 54s | Max: 15m 06s
    🟩 GCC                Pass: 100%/9   | Total: 52m 05s | Avg:  5m 47s | Max: 20m 38s
    🟩 MSVC               Pass: 100%/2   | Total: 23m 02s | Avg: 11m 31s | Max: 11m 33s | Hits:  69%/312   
    🟩 NVHPC              Pass: 100%/2   | Total: 12m 04s | Avg:  6m 02s | Max:  6m 18s
  🟩 gpu
    🟩 v100               Pass: 100%/26  | Total:  2h 31m | Avg:  5m 48s | Max: 20m 38s | Hits:  69%/312   
  🟩 jobs
    🟩 Build              Pass: 100%/24  | Total:  1h 55m | Avg:  4m 48s | Max: 11m 33s | Hits:  69%/312   
    🟩 Test               Pass: 100%/2   | Total: 35m 44s | Avg: 17m 52s | Max: 20m 38s
  🟩 sm
    🟩 90                 Pass: 100%/1   | Total:  3m 27s | Avg:  3m 27s | Max:  3m 27s
    🟩 90a                Pass: 100%/1   | Total:  3m 42s | Avg:  3m 42s | Max:  3m 42s
  🟩 std
    🟩 17                 Pass: 100%/6   | Total: 23m 50s | Avg:  3m 58s | Max:  5m 46s
    🟩 20                 Pass: 100%/20  | Total:  2h 07m | Avg:  6m 21s | Max: 20m 38s | Hits:  69%/312   
  

👃 Inspect Changes

Modifications in project?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
+/-	CUDA Experimental
	python
	CCCL C Parallel Library
	Catch2Helper

Modifications in project or dependencies?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
+/-	CUDA Experimental
	python
	CCCL C Parallel Library
	Catch2Helper

🏃‍ Runner counts (total jobs: 26)

#	Runner
18	linux-amd64-cpu16
4	linux-arm64-cpu16
2	windows-amd64-cpu16
2	linux-amd64-gpu-v100-latest-1