Add Pillow dependency

[JoelNiklaus]

Add Pillow Dependency

Problem

Running lighteval accelerate can fail during task registry initialization with ModuleNotFoundError: No module named 'PIL'.

Root Cause

The built-in MathVista task imports PIL.Image, but Pillow is not declared in the project dependencies.

Solution

Add pillow to the core project dependencies so built-in task discovery has the package it imports.

[bot-ci-comment]

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