make a singleton thread pool executor for tinygrad since we always want it to run on the same thread

exo/inference/tinygrad/inference.py

@@ -61,12 +61,13 @@ def build_transformer(model_path: Path, shard: Shard, model_size="8B", device=No
 
   return model
 
+_executor = ThreadPoolExecutor(max_workers=1) # singleton so tinygrad always runs on the same thread
 class TinygradDynamicShardInferenceEngine(InferenceEngine):
   def __init__(self, shard_downloader: ShardDownloader):
     self.shard = None
     self.shard_downloader = shard_downloader
-    self.executor = ThreadPoolExecutor(max_workers=1)
     self.states = OrderedDict()
+    self.executor = _executor
 
   def poll_state(self, x, request_id: str, max_states=2):
     if request_id not in self.states: