Remove the single process, multi-gpu feature.

[joaander]

Description

Remove the single process, multi-gpu feature.

Using Multiple GPUs via MPI domain decomposition is still supported.

Motivation and context

Resolves #1664

How has this been tested?

CI checks.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.
• I have summarized these changes in CHANGELOG.rst following the established format.

[mphoward]

@joaander I saw this PR open up (great!) and had a quick question: what does removing this code path mean for GlobalArray vs. GPUArray for developers in current & future code? I remember the GlobalArray pattern being rather complicated (CRTP) but in the end, I think degrading to essentially just a GPUArray if there is only 1 GPU per process?

hoomd-blue/hoomd/GlobalArray.h

Lines 13 to 16 in ec0b84c

	GlobalArray<> supports all functionality that GPUArray<> does, and should eventually replace
	GPUArray. In fact, for performance considerations in single GPU situations, GlobalArray
	internally falls back on GPUArray (and whenever it doesn't have an ExecutionConfiguration). This
	behavior is controlled by the result of ExecutionConfiguration::allConcurrentManagedAccess().

[joaander]

I am planning to leave GlobalArray in place for now. A future PR might remove it (and GlobalVector). Care must be taken in cases where force_managed is set to true:

hoomd-blue/hoomd/GlobalArray.h

Lines 224 to 236 in ec0b84c

	bool force_managed = false)
	: m_exec_conf(exec_conf),
	#ifndef ALWAYS_USE_MANAGED_MEMORY
	// explicit copy should be elided
	m_fallback((exec_conf->allConcurrentManagedAccess()
	|| (force_managed && exec_conf->isCUDAEnabled()))
	? GPUArray<T>()
	: GPUArray<T>(num_elements, exec_conf)),
	#endif
	m_num_elements(num_elements), m_pitch(num_elements), m_height(1), m_acquired(false),
	m_tag(tag), m_align_bytes(0),
	m_is_managed(exec_conf->allConcurrentManagedAccess()
	|| (force_managed && exec_conf->isCUDAEnabled()))

These cases will need to maintain the use of managed memory.

In all cases where force_managed = false (the default), GlobalArray is GPUArray now. Future code should continue to use GPUArray in all cases that managed memory is not necessary.

[joaander]

It would appear that force_managed is never set true. In that case, maybe the GlobalArray / GlobalVector removal is coming soon as it is primarily a global search and replace operation.