ops: sync the CPU with only the offload stream activity

This was syncing with the offload stream which itself is synced with the
compute stream, so this was syncing CPU with compute transitively. Define
the event to sync it more gently.

comfy/model_management.py

@@ -1196,7 +1196,10 @@ def get_pin_buffer(offload_stream):
         pin_buffer = comfy_aimdo.host_buffer.HostBuffer(0)
         STREAM_PIN_BUFFERS[offload_stream] = pin_buffer
     elif offload_stream is not None:
-        offload_stream.synchronize()
+        event = getattr(pin_buffer, "_comfy_event", None)
+        if event is not None:
+            event.synchronize()
+            delattr(pin_buffer, "_comfy_event")
     return pin_buffer
 
 def resize_pin_buffer(pin_buffer, size):

comfy/ops.py

@@ -241,6 +241,7 @@ def cast_modules_with_vbar(comfy_modules, dtype, device, bias_dtype, non_blockin
             pin = stream_pin_tensor[pin_offset:pin_offset + pin_size]
             cast_maybe_lowvram_patch(xfer_source, pin, None)
             comfy.model_management.cast_to_gathered([ pin ], xfer_dest, non_blocking=non_blocking, stream=offload_stream)
+        stream_pin_hostbuf._comfy_event = offload_stream.record_event()
 
     return offload_stream