Fix PIL image hashing to use actual bytes instead of object repr (#3331)

The convert_pil_to_hash function was hashing str(BytesIO) which includes
the memory address, causing different hashes for the same image across
runs. This made task hashes non-deterministic for tasks with images.

Reproducer:
```python
import hashlib
from io import BytesIO
from PIL import Image

img = Image.new('RGB', (2, 2), color='red')
img_bytes = BytesIO()
img.save(img_bytes, format="PNG")

# Buggy: hashes "<_io.BytesIO object at 0x...>"
print(str(img_bytes))  # <_io.BytesIO object at 0x1023d8bd0>
buggy_hash = hashlib.sha256(str(img_bytes).encode()).hexdigest()

# Fixed: hashes actual PNG bytes
print(img_bytes.getvalue()[:30])  # b'\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x02...'
fixed_hash = hashlib.sha256(img_bytes.getvalue()).hexdigest()
```

Running the same image twice:
- Buggy approach: different hashes each time due to memory address
- Fixed approach: consistent hash de33ddc09a0ba9b8...

This fix ensures deterministic task hashes for evaluations with images.

Author: tboerstad

lm_eval/utils.py

@@ -576,7 +576,7 @@ def convert_pil_to_hash(value):
 
     img_bytes = BytesIO()
     value.save(img_bytes, format="PNG")
-    return hashlib.sha256(str(img_bytes).encode()).hexdigest()
+    return hashlib.sha256(img_bytes.getvalue()).hexdigest()
 
 
 def convert_bytes_to_hash(value):