Support for async execution functions

This commit adds support for node execution functions defined as async. When
a node's execution function is defined as async, we can continue
executing other nodes while it is processing.

Standard uses of `await` should "just work", but people will still have
to be careful if they spawn actual threads. Because torch doesn't really
have async/await versions of functions, this won't particularly help
with most locally-executing nodes, but it does work for e.g. web
requests to other machines.

In addition to the execute function, the `VALIDATE_INPUTS` and
`check_lazy_status` functions can also be defined as async, though we'll
only resolve one node at a time right now for those.

comfy_execution/graph.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 from typing import Type, Literal
 
 import nodes
+import asyncio
 from comfy_execution.graph_utils import is_link
 from comfy.comfy_types.node_typing import ComfyNodeABC, InputTypeDict, InputTypeOptions
 
@@ -100,6 +101,8 @@ class TopologicalSort:
         self.pendingNodes = {}
         self.blockCount = {} # Number of nodes this node is directly blocked by
         self.blocking = {} # Which nodes are blocked by this node
+        self.externalBlocks = 0
+        self.unblockedEvent = asyncio.Event()
 
     def get_input_info(self, unique_id, input_name):
         class_type = self.dynprompt.get_node(unique_id)["class_type"]
@@ -153,6 +156,16 @@ class TopologicalSort:
         for link in links:
             self.add_strong_link(*link)
 
+    def add_external_block(self, node_id):
+        assert node_id in self.blockCount, "Can't add external block to a node that isn't pending"
+        self.externalBlocks += 1
+        self.blockCount[node_id] += 1
+        def unblock():
+            self.externalBlocks -= 1
+            self.blockCount[node_id] -= 1
+            self.unblockedEvent.set()
+        return unblock
+
     def is_cached(self, node_id):
         return False
 
@@ -181,11 +194,16 @@ class ExecutionList(TopologicalSort):
     def is_cached(self, node_id):
         return self.output_cache.get(node_id) is not None
 
-    def stage_node_execution(self):
+    async def stage_node_execution(self):
         assert self.staged_node_id is None
         if self.is_empty():
             return None, None, None
         available = self.get_ready_nodes()
+        while len(available) == 0 and self.externalBlocks > 0:
+            # Wait for an external block to be released
+            await self.unblockedEvent.wait()
+            self.unblockedEvent.clear()
+            available = self.get_ready_nodes()
         if len(available) == 0:
             cycled_nodes = self.get_nodes_in_cycle()
             # Because cycles composed entirely of static nodes are caught during initial validation,