[REFACTOR] PassContext.fallback_device -> PassConfig.config

[tqchen]

#5631 Introduces a generic config mechanism to store the pass configuration options. Given that fallback_device is a pas related config, We should probably also migrate PassContext->fallback_device to a special config. e.g. relay.fallback_device_type with IntImmNode as its node type

[tqchen]

cc @zhiics @comaniac

[zhiics]

@tqchen Thanks for reminding. I will take a stab next week.

[zhiics]

I realize we may also want to remove relay.build_config and directly use PassContext to keep it consistent to tir, but it may invoke a lot of changes. @tqchen thoughts?

[tqchen]

I agree we should do that as well

[zhiics]

One potential problem is that it may break downstream deployment.

[tqchen]

We could mark build_config as deprecation, and remove that API in the next release cycle. In the meanwhile, we should replace all of the current usages.

[zhiics]

Sounds like a good plan.