[BUG]: Legacy/per-thread default streams should be singletons

[leofang]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Type of Bug

Runtime Error

Component

cuda.core

Describe the bug

When #1483 started I missed that we wanted to expose them as singletons:

cuda-python/cuda_core/cuda/core/_stream.pyx

Lines 420 to 422 in 264524b

	# standard python objects, public
	LEGACY_DEFAULT_STREAM = C_LEGACY_DEFAULT_STREAM
	PER_THREAD_DEFAULT_STREAM = C_PER_THREAD_DEFAULT_STREAM

whereas the public methods exposed by #1483 would always return new Python instances. We should fix it before v0.6.0 code freeze.

How to Reproduce

n/a

Expected behavior

n/a

Operating System

No response

nvidia-smi output

No response

[Monishver11]

Hello @leofang, I've fixed this bug. Could you please review PR #1496? Thanks.