Fix Pickling Windows

[Katehuuh]

Modify #34 (comment) for current version. Fix the issue for me.

diff Fix Pickling Windows

setup.py:

    install_requires=[
+        'dill',

stable_audio_tools/data/dataset.py:

+import dill

-self.custom_metadata_fns[config.path] = config.custom_metadata_fn
+self.custom_metadata_fns[config.path] = dill.dumps(config.custom_metadata_fn)

-                    custom_metadata_fn = self.custom_metadata_fns[custom_md_path]
-                    custom_metadata = custom_metadata_fn(info, audio)
+                    custom_metadata_fn_deserialized = dill.loads(self.custom_metadata_fns[custom_md_path])
+                    custom_metadata = custom_metadata_fn_deserialized(info, audio)

[ghost]

Can confirm this fixed the issue on Windows 10.