Tpetra: Deprecate and remove DefaultPlatform, MpiPlatform, and SerialPlatform

[mhoemmen]

@trilinos/tpetra

Expectations

Tpetra should have a single source for initialization and for default values of template parameters.

Motivation and Context

Tpetra::DefaultPlatform is a hold-over from an old Tpetra initialization model that included something called HybridPlatform. Tpetra now has different plans for truly hybrid (e.g., GPU + CPU concurrently) parallelism, that the "platform" concept would not support. Furthermore, Platform is tied to Node, but we want to get rid of Node (#57).

Definition of Done

• Replace all use of Tpetra::DefaultPlatform to initialize Tpetra with Tpetra::ScopeGuard
• Replace all use of Tpetra::DefaultPlatform::DefaultPlatformType::NodeType with Tpetra::Map<>::node_type
• Deprecate Tpetra::DefaultPlatform, Tpetra::MpiPlatform, and Tpetra::SerialPlatform
• Remove Tpetra::DefaultPlatform, Tpetra::MpiPlatform, and Tpetra::SerialPlatform

Related Issues

• Blocked by Panzer: Implicit dependence on Tpetra::DefaultPlatform include via Teko #3119
• Blocks Tpetra: Replace Node with Kokkos space #57

[mhoemmen]

I would like to clarify my comment here that "Tpetra::*Platform is the last left-over of an idea for hybrid parallelism that likely would have been better served by MPI intercommunicators." What I meant is that HybridPlatform assigned different kinds of thread parallelism to different MPI processes within the same MPI communicator. This makes load balancing harder and doesn't address the issue that a single MPI process may have access to different thread-parallel hardware resources. One could achieve the goal of HybridPlatform by creating different MPI communicators for different kinds of hardware resources, and possibly joining them via intercommunicators.