Pin transformers to 4.56 line for MiniCPM4.1-8B compatibility

The latest transformers removed is_torch_fx_available from
transformers.utils.import_utils, which MiniCPM4.1-8B's bundled modeling code
imports, breaking model load on the Space. Pin to >=4.56,<4.57 (the version
OpenBMB documents) which still provides that symbol.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements.txt

@@ -1,5 +1,7 @@
 gradio
 spaces
-transformers>=4.51.0
+# MiniCPM4.1-8B's bundled modeling code imports is_torch_fx_available, which
+# newer transformers removed; pin to the 4.56 line OpenBMB tested against.
+transformers>=4.56,<4.57
 torch
 accelerate