mm: reset_cast_buffers: sync compute stream before free (#12822)

Sync the compute stream before freeing the cast buffers. This can cause
use after free issues when the cast stream frees the buffer while the
compute stream is behind enough to still needs a casted weight.

Author: rattus128

comfy/model_management.py

@@ -1148,6 +1148,7 @@ def reset_cast_buffers():
     LARGEST_CASTED_WEIGHT = (None, 0)
     for offload_stream in STREAM_CAST_BUFFERS:
         offload_stream.synchronize()
+    synchronize()
     STREAM_CAST_BUFFERS.clear()
     soft_empty_cache()