CPU fallback hint

[anssiko]

The group has explored a "CPU fallback" hint, a flag to expose to web content whether a CPU fallback mechanism is active. For background, see related discussion links below.

While the group reached consensus on the accelerated hint (issue #815 fixed by PR #895), we did not yet reach consensus on the CPU fallback mechanism part of the proposal.

This separate issue is to focus our attention on this possible future enhancement, building on the foundation established in #895.

Explainer:

• https://github.com/webmachinelearning/webnn/blob/main/device-selection-explainer.md

Related discussion:

• Query supported devices before graph compilation #815
• Add a simple accelerator selection mechanism. #895
• Define "underlying execution device" concept #897