Pin transformers<4.40.0 for PyTorch 2.1 compatibility

transformers 4.40+ uses serialized_type_name kwarg in _register_pytree_node()
which doesn't exist in PyTorch 2.1. Pin to older version.

Also removes the ineffective pytree shim.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

requirements.txt

@@ -1,9 +1,13 @@
-# Core ML - Pin to AudioCraft-compatible versions (2.4+ breaks pytree API)
+# Core ML - Pin to AudioCraft-compatible versions
 torch>=2.1.0,<2.4.0
 torchaudio>=2.1.0,<2.4.0
 audiocraft>=1.3.0
 xformers>=0.0.22
 
+# Pin transformers to version compatible with PyTorch 2.1
+# Newer versions use serialized_type_name kwarg not in PyTorch 2.1
+transformers>=4.30.0,<4.40.0
+
 # UI
 gradio>=4.0.0