Adds monarch.actor_mesh.stack

[allenwang28]

Summary:
Context - one use case coming up in forge work is that we might want some pattern like this:

class Environment(Actor):
    endpoint
    def step(self, action):
        return "step"


class BrowserEnv(Environment):
    endpoint
    def step(self, action):
        return "browser"


class CodingEnv(Actor):
    endpoint
    def step(self, action):
        return "coding"


m = proc_mesh(...)
browsers = m.spawn("browser", BrowserEnv)
coders = m.spawn("coder", CodingEnv)

Previous APIs that would require:

for a in [browsers, coders]:
    a.step.call().get()

etc.

This change supports:

envs = monarch.stack(browsers, coders)

which enables

envs.step.call().get()

Implementation specific details:

• Introduces StackedActorMeshRef and StackedEndpoint
  • These are essentially thin wrappers to ActorMeshRef and Endpoint but inherit the same APIs
• Introduces EndpointInterface for which Endpoint and StackedEndpoint both inherit from.
• stack can automatically detect the closest ancestor in its inheritance tree, or allow the user to provide a common interface (i.e. if the user prefers a specific common ancestor)

Differential Revision: D77606019

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D77606019